Move listers to powerline.listers.vim module
Make listers accept any number of keyword arguments
Do not show unlisted buffers

Closes #972
Update mode in segment_info in tabpage lister
Remove duplicate information from configuration/segments.rst
Also show documentation for plugin-specific segments
Add lister reference
Use generators in place of list comprehension in listers
Add documentation about creating listers
Remove nonlocal image URLs from overview
Use buf_nc and tab_nc modes in listers
Fix incorrect description of draw_inner_divider
Remove duplicate documentation of draw_inner_divider
Add support for `draw_inner_divider` in listers
Add bufnr and modified_indicator segments to tabline
Fix exclude_/include_modes support with new segment['mode'] key
Merge pull request #993 from ZyX-I/listers

Some lister improvements

Fixes #989
Closes #972
Ref #614