Add completion modes for words and WORDs

Default mode: `word`. Set this to complete WORDs instead:
```vim
:let g:tmux_complete_mode='WORD'
```
Example on:
```c
string url = "http://go
```
- mode `word` completes on `go`
- mode `WORD` completes on `http://go`

Also removes surrounding non-word characters like the double quote in the example above.

As discussed in https://github.com/wellle/tmux-complete.vim/pull/17#issuecomment-38033747. Close #19.