Clean up mappings: LocalLeader, Plug, and noremap

This is a set of changes to convert vim-ipython to use common vim plugin conventions:

* Use LocalLeader for ftplugins - so users can define filetype-specific and global mapleaders.
* Use `<Plug>` mappings so all mappings can be defined by users (even ones that use `<SID>` functions). This makes it easier to make internal changes (like renaming python functions) without breaking user's mappings.
* Use noremap where possible. Ensures that plugin commands run as expected. As far as I understand, you can't use noremap when mapping to Plug, but maps that start with Plug will never evaluate to a user's sequence of keys, so that shouldn't be a problem.

I tried to break it up into small changes to make the diffs easier to read.