Change paste mode toggle shortcut

Using <Leader><Leader> as a shortcut didn't really make much sense. Thus
this was changed to <Leader>p, for "p"aste.
Allow revision to be specified in plugins.list

In order to allow plugins to be frozen at a particular revision, the
plugins.list file now supports a second item, representing the revision
to freeze the plugin at.

This also becomes useful for plugins like vim-powerline, which don't
develop on the traditional `master` branch, but instead use `develop`.
Don't hard-reset if remote hasn't changed

When running the `update` script, it was annoying to see a line for each
plugin appear as a result of executing `git reset --hard $revision`, as
it made it hard to see which plugins actually changed.

This changes the behaviour to hard-reset only when the remote has
changed.
Don't fetch origin if HEAD matches plugin revision

Previously, if you ran the `update` script and a plugin was defined with
an explicit hash, it would still attempt to fetch changes from the
plugin's upstream origin.

Since having an explicit hash defined for a plugin means that this fetch
never needs to happen once the plugin's `HEAD` has that hash, this
commit optimizes the update script to skip the fetch step in the event
the revision hash is defined and it matches the current plugin's `HEAD`.
Match sign column background to line number column

This sets the `SignColumn` background colour to be the same as the
`LineNr` background colour so that they appear as one.
Don't show line numbers in git commit messages

Since these messages are mostly prose, there's no need to have line
numbers listed.
Add extra <CR> to some shortcut commands

This prevents the annoying "Press ENTER or type command to continue"
prompt from appearing when these commands are executed in an empty Vim
window.

We can't just disable the prompt because it is useful in other
situations, for example warning before closing a file that has been
modified.

Thus we add an extra <CR> to the problematic shortcuts so that the
message is auto-dismissed.
Change window-splitting shortcuts to match tmux

This further reinforces the parallels in shortcuts between tmux and vim.
Enable paste buffer yanking to local clipboard

Using Clipper (https://wincent.com/products/clipper) and combining it
with Ctrl-Y shortcut key, the current paste buffer can now be yanked to
the local machine's clipboard.

In the event Clipper isn't running, this fails gracefully with a bell
alert.
Replace <Enter> with <CR>

`<CR>` is shorter and is recognized by the syntax highlighter.
Add Extradite plugin for viewing git log of file

Adds a slick interface for browsing the git log history of the current
file.