Improve gO outline

[liquidev]

The current outline is pretty lackluster:

As shown in the screenshot, it's not easy to tell overloads apart, and object fields and local variables add a lot of noise and make the outline harder to navigate.

Here are my suggestions on improving it:

• allow the user to hide certain parts of the outline, eg. fields and local variables, perhaps using a simple config system like let g:nim#outline_hide_fields = 0 or let g:nim#outline_hide = ["fields", "localvars"]
• show proc/iterator/template/macro signatures and var/let/const/field types