Update readme to ref inlined tmux conf expressions

Update the readme to describe inlined expressions in the tmux map which lets us avoid the need for the external script.

My only reservation is that I have found that newer versions of vim cause the tmux `pane_current_command` expression to return a title cased 'Vim'. I have had to hard code the tmux.conf expressions to match.

I would like to provide a more general, case insensitive match expression, but I can't seem to sort it out.

``` tmux
bind -n C-h if "[[ $(tmux display -p '#{pane_current_command}') =~ '[vV][iI][mM]' ]]" "send-keys C-h" "select-pane -L"
```

@derekprior, Do you have any suggestions on how to get the maps working case insensitively?