Remove keypad() invocation from resize_all()

Seems to be useless, one call in setup_curses_interface() should be
enough.
Automatically resize pos window to fit text

Addresses first part of #55 at GitHub.

Thanks to ranousse.
Add %[ %] optional markers for statusline

The second part for #55 on GitHub.
Remove digits from statusline tests
Enable restricting set of macros

This allows for merging ruler and statusline expansions.
Reuse statusline macros parsing for the ruler
Update 'rulerformat' default value

In compatible way.

 - Fix documentation which specified "-" separator instead of "/" in
   code.

 - Remove leading "%=", which is effectively useless because of the
   ruler being right-aligned by default.
Document %[ and %] macros (optional blocks)
Rename "position window" to ruler in code
Rename cfg.{ last_status => display_statusline }

"last_status" is hard to reason about when looking at the code.
Rename update_ruler() => ui_ruler_update()
Add empty lines to ui/ui.h

For better readability.
Rename view_draw_pos() -> view_ruler_update()
Merge branch 'update-ruler-statusline-formatting'

Adds optional macro expansion to statusline/ruler via %[ and %] macros.

Thanks to ranousse.

Closes #55 at GitHub.