[Feature] Support multikey commands

[terryma]

Currently the plugin doesn't work for multikey commands in any mode once multicursor mode is started. Actions like ciw in normal mode causes undetermined behavior since the plugin literally tries to fan out c, i, and w individually at every virtual cursor location. Adding this support requires the plugin to be more intelligent with user input, deciding what to 'keep' vs what to 'fan out'.

[eapache]

This is basically already done by the insert_maps and normal_maps. The only thing left is visual_maps (ticket #111) and then setting some sane defaults for these options.

My .vimrc currently contains the following, which I suggest as a good starting point for defaults.

let g:multi_cursor_insert_maps={'\':1}
let g:multi_cursor_normal_maps={'c':1, 'd':1, 'f':1, 'r':1, '=':1, '<':1, '>':1}

[roryokane]

I think this is a complete list of Vim multi-key mappings, and should be the default for multi_cursor_normal_maps:

let g:multi_cursor_normal_maps={'!':1, '@':1, '=':1, 'q':1, 'r':1, 't':1, 'T':1, 'y':1, '[':1, ']':1, '\':1, 'd':1, 'f':1, 'F':1, 'g':1, '"':1, 'z':1, 'c':1, 'm':1, '<':1, '>':1}

I identified these multi-key mappings by going across the keyboard, row by row, and checking whether that key, or Shift plus that key, was a multi-key mapping by default.

[faceleg]

@roryokane do you want to submit a PR with this change, assuming it won't break anything?