Show various marks in the left margin.

Show various marks in the left margin.

Use signs to highlight the location of the following marks (see help on
'x for information on mark x): a-f A-F .  ' " ^ < > [ ]
Toggle marks with \m.
Update marks when entering/leaving insert mode and when CursorHold is
triggered.  Trigger CursorHold after 500ms to reduce lag.
Show marks and numbers by default.  Set terminal mouse mode to allow
copying text from Vim to work as expected with the left margin showing
stuff.
Make sure the window isn't resized every time .vimrc is sourced.
Split autocommands into groups to help keep track of all this stuff..