Update Git .mailmap file

To consolidate authorship info as emitted by `git shortlog -s`.
Implement ctrl-w

Originally submitted as:

  https://github.com/wincent/Command-T/pull/11

Changes:

- rebased and fixed merge conflicts (for example, the changes to
  plugin/command-t.vim actually needed to be applied to
  autoload/commandt.vim)
- use `list_matches!` instead of `list_matches` (necessary because
  input debouncing was added since the original submission of the pull
  request)
Add Kevin Webster to Git .mailmap file
Update docs with info about <C-w> feature

- add Kevin Webster to AUTHORS list
- document option for configuring the mapping
  (`g:CommandTClearPrevWordMap`)
- update HISTORY section
Tweak word clearing to match shell behavior

The word clearing algorithm was clearing either words or whitespace. For
example, given the following (where ^ indicates the cursor position):

  foo      bar     baz
                      ^

Hitting <C-w> three times would first delete "baz", then the whitespace
after "bar", finally "bar" itself.

This commit changes the behavior to match that of the shell:

  foo bar
              ^ (<C-w> deletes trailing whitespace and "bar")
  foo       bar
        ^ (<C-w> deletes "foo" and whitespace to left of cursor)
  foobar
     ^ (<C-w> deletes "foo")
  foo
  ^ (<C-W> does nothing)

For clarity and simplicity, switched to a largely regex-based approach
which has no conditionals in it and is probably more readable.
Merge branch 'rabidpraxis/implement-ctrl-w' into next

* rabidpraxis/implement-ctrl-w:
  Tweak word clearing to match shell behavior
  Update docs with info about <C-w> feature
  Add Kevin Webster to Git .mailmap file
  Implement ctrl-w