Add shortcut for magic regex syntax in Vim

Using \v at the beginning of a Vim search saves you from having to type
a lot of escape characters.

This commit maps the search key / to auto-append \v.
Automatically reload file if changed elsewhere

Frequently when editing a file and rebasing/merging in `git`, I would
attempt to save my changes to a file only to discover that the file has
been touched by `git`.

This attempts to combat the issue by running the `checktime` command
on certain events (e.g. entering/leaving insert mode).

`checktime` automatically detects if the file has changed and displays
an appropriate prompt, making this work just like before but _before_ I
make a bunch of changes.
Document some configuration settings
Increase max number of CtrlP suggestions to 12

Since the buffers I open usually have at least 20 lines to work with,
I've increased the maximum number of items that can be suggested by
CtrlP.