Added extension for tpope/vim-capslock

Hi

I have added support for [vim-capslock](https://github.com/tpope/vim-capslock).
This extension displays CAPS when the soft caps lock is enabled.

I locked at how the other extensions was made and tried to do something similar.
Please let me know if you want something done another way.

I had to use `airline#parts#define_text` and not `airline#parts#define_empty` or else an extra arrow would show when vim-capslock is not present. Not sure if this is correct.