Provide option to unbind default <Tab> and <CR> bindings for abbreviation expansion

Hello!

I'm a big fan of the plugin and have been using it for class.

I would like the option to unbind `` and `` from expanding abbreviations, _without_ also disabling abbreviations entirely. If someone (like me) has an insane personal preference for using keys other than `` and `` for completion, it can be annoying to hit `` for indenting the line and then get an expansion instead.

It is currently easy to map another key to **expand** abbreviations, using the `require("lua.abbreviations").convert()` function. I believe it would also be nice to either:

1. Allow for `keymaps = false` in `config.abbreviations` in order to not bind `` and ``, or
2. Allow for `config.mappings = false` to remove those bindings.

Thank you -- your plugin has really saved my butt in my Formal Methods class becuase now I don't need to leave Neovim to do my coursework!