[feature] Improve jumping between function/module definitions.

In addition to #390, they can be improved to provide the following:
* ]m - go to start of next function definition.
* ]M - go to end of next function definition.
* [m go to start of previous function definition.
* [M go to end of previous function definition.
* ]] go to start of next module definition.
* ][ go to end of next module definition.
* [[ go to start of previous module definition.
* [] go to end of previous module definition.

Once again stolen from [here](https://github.com/vim-ruby/vim-ruby/blob/71f5df78a45c5458da793b2c897ff5dcdcd9a819/doc/vim-ruby.txt#L10)

Those will add more control to you since the current behaviour is only:
- ]] - jump to beginning of next definition do/end
- [[ - jump to previous definition of a do/end

You lose control of jumping to the `end` part.
It might be pedantic to add different support for modules/functions.