Add [count]gbc keymap

[kuznetsss]

Hi! Thank you for the plugin, it's really useful!

I'm really missing the feature to toggle block comment for multiple lines using [count]gbc keymap. What do you think about adding it?

I know that there is gb[count]{motion} but for me it's less comfortable

[numToStr]

I am already planning to add this after I merge this #94 :)

[numToStr]

Wanna try #97