Prompt extension

As discussed in #389, this is the airline-promptline extension. Please let me know if you think the code can be improved.

This is how it works:
- whenever `add_theme_func`-registered functions are executed, this extension calls `promptline#api#create_snapshot_with_theme(file, theme)`
- `promptline#api#create_snapshot_with_theme(..)` creates a shell file which should be sourced on shell login

Currently, the end-user has to set `airline#extensions#promptline#snapshot_file`, otherwise this extension will not be enabled.

Also, please don't merge the PR yet. I still have to:
- create a showcase png/giff for the README
- improve error handling in `promptline#api#create_snapshot_with_theme` (if the file isn't writable, an 'ugly' error will be displayed)

I'm open for any suggestions for improvement :) Close #389