Bash: add SSH and SCP shortcut for my VPS

Firstly, this requires that the MT_URL and MT_PORT environmental
variables are set before ~/.shell_commands is sourced. These values
contain potentially sensitive information, so they're kept out of source
control directly. Currently, they're set in ~/.shell_private.

In ~/.shell_commands, we add an alias, `mt.com`, which SSH'es into my
VPS.

In the same file, we also add a wrapper around `scp` which detects if
`mt.com` is one of the URLs and replaces it with the real VPS URL. If no
port was specified in the user's scp arguments, it also adds the port of
my VPS to the scp arguments. Thus, the command `scp mt.com:~/a_file.txt
./` will correctly download a_file.txt from my VPS, into the local dir.
(This is accomplished by writing a function which mangles the args to
replace the URL and/or port, then calls `scp` with those args.)
Bash: fix bug in 'mt.com' alias

Was using the wrong name of the VPS URL and port variables. It's now
fixed to use the right ones.
Merge commit '2124464ce' into Linux

Backports shortcuts for connecting to my VPS.

Conflicts:
	home/.shell_commands
Vim: fix format options in git commit msgs

Our normal formatoptions were getting overridden by the git language's
gitcommit filetype handler. This was causing weird behavior, like the
first line being concatenated with the first line of the git changes
comment block as soon as you started typing; and difficulty inserting
newlines between some characters.

This fix re-sets the formatoptions to our preferred ones in our user
gitcommit file handler in ~/.vimrc.after.

We also change the color column in gitcommit messages to match the
textwidth. This will allow us to see, visually, where our text will be
wrapped.
Git: make path to ~/.gitsupport relative

Before, we hard-coded the full path to our User directory. This commit
changes it to use "~/" instead, which should allow it to be used on
a wider range of machines.
Vim: Add 'w' to format options

'w' causes Vim to assume lines with whitespace at the end continue a
paragraph. While we normally don't want whitespace at the end of lines,
maybe this will help with the "can't insert a newline into the middle
of a comment" bug (where, with 'a' in the formatoptions, Vim assumes
the two new lines are part of the same paragraph and immediately
autoformats them back into a single word-wrapped paragraph.)

Just to test it out. May revert later.
Vim: add command to toggle 'a' in formatoptions

`\a` is the keyboard shortcut in normal mode. On Vim startup, 'a' will
be in the formatoptions. Pressing \a will remove it; pressing \a again
will add it back again.

This is a bandaid fix for the problem of not being able to insert a
newline in the middle of a comment or other wrapped text. This is
because Vim tries to immediately autoformat the text, and (since there
isn't a blank line between the two lines, and Vim therefore thinks the
two lines are part of the some paragraph,) joins the two lines back
together, negating the newline we just entered.

With this change, we can toggle 'a' off, insert a newline and reformat
the text manually, then reenable 'a'.

Read the TODO in the comments for more of my thoughts on this.
Bash: only add /usr/local/bin to PATH if it exists

This should allow this .bash_profile to work on machines without
homebrew (first steps toward a unified set of dotfiles between Mac and
Linux.)
Bash: set $OS based on current system

The first thing ~/.bash_profile will now do is detect the operating
system of the system it's running on, and set the globally exported
variable $OS to either "Mac" or "Linux".

This is part of our (still early) work to use the same dotfile branch
on both Mac and Linux. OS-specific configuration can now be guarded by
a `if [[ $OS == Mac ]]; then` (or 'Linux'), allowing for the same file
to be used on both systems without errors.
Merge commit 'af6b175a21b75c7e' into Linux

This backports our changes to Vim regarding formatoptions. It also backports our changes to ~/.bash_profile for detecting the OS. This also uncomments adding some Mac-specific directories to the $PATH; however, since they're guarded by `if [[ -d ... ]]`, those shouldn't interfere with Linux.

Conflicts:
	home/.bash_profile