Fix #54 allow users to filter out panes based on arbitrary strings

[eyalfir]

Enable blacklisting pane titles in a configurable way.

[slashfoo]

hey @eyalfir thanks for the awesome suggestion, I'd like to suggest a few changes.

1. rename the variable to something more meaningful like g:slimux_blacklist_words
2. make the variable a list
3. Instead of using grep -v use grep -vF and pass it as a parameter a \n concatenated list of the patterns specified. -F makes it so that these are interpreted as plain words, I think using words rather than regex would be good for now.
4. Add a blurb about it in the README.md.

If you'd rather I did the changes or if you'd need anything, let me know.

[sunaku]

Couldn't this be done in a user's Vim configuration by registering an autocmd to run upon entering the slimux pane selection window? The autocmd would (1) set modifiable and (2) use the global command g/blacklist_regex/d to filter out lines the user doesn't want to see.