Remove git email filter

This wasn't really being used (the currently configured email in git
is, in fact, my public GitHub email, and the only other use of my
private email was in ~/.hgrc. I changed that file to use my public
email, and now the email filter is redundant.

It also wasn't really implemented right, so that's another reason to
toss it.
Git: create common gitconfig and include system-specific config

Previously, ~/.gitsupport/gitconfig.{mac,linux} were independent,
self-contained files that had all the git configuration to run on Mac
or Linux. This meant there was a lot of duplicate configuration between
the two. We then manually symlinked one or the other to ~/.gitconfig.

We now add a ~/.gitconfig file to the repo containing all the common
configuration. It has an include directive for
~/.gitsupport/gitconfig.active, which should be manually symlinked to
~/.gitsupport/gitconfig.mac or ~/.gitsupport/gitconfig.linux. These two
OS-specific files contain just the configuration specific to that OS
(for example, the Mac one has configuration for SourceTree.)
.homesickrc will now symlink the right active gitconfig + hide ~/bin

We expand the existing .homesickrc Ruby script that homesick runs when
symlinking a new dotfile repo. It will now check which OS it's running
on, and symlink ~/.gitsupport/gitconfig.{mac,linux} to
~/.gitsupport/gitconfig.active (which is included in ~/.gitconfig). It
then will, if running on Mac OS X, run `chflags hidden ~/bin`.
bash: remove custom ssh completion (bash-completion2 does same now)

~/.bash_completion previously contained bash auto-completion for the
`ssh` command which scraped ~/.ssh/known_hosts for server addresses and
auto-completed the ssh address based on those. However, the homebrew
package `bash-completion2` (as opposed to the old `bash-completion`)
now includes a `ssh` auto-completion function which does the same and
more (like scraping ~/.ssh/config for addresses.)

We therefore remove our own, custom `ssh` completion in favor of the
standard one.
bash: remove $GREP_OPTIONS in favor of `grep` alias for colors

Grep was complaining that $GREP_OPTIONS is now depreciated. It
recommends using an alias to `grep` (or function) that adds the options
we want. So we create such an alias that adds the `--color=auto` option
to the grep command automatically.
bash: add `mkcd` command which created a directory & `cd`s to it
Vim: use spaces instead of tabs

It's the JavaScript standard, and allows Vim Indent Guides to work
better. So why fight it?
Bash: add canonical Araxis Merge CLI tools to $PATH, not our copy

Previously, to enable support for Araxis Merge in the terminal, we
simply copied all the CLI binaries from the Araxis package to
~/bin/araxis. However, the currently recommended way to do this is
simply to add the path to the tools inside the Araxis Merge app bundle
to the $PATH. This commit does that.
Git: add '.idea' folders to global gitignore