Add padding spaces for section in s:get_section.

When section text is non-empty, pad spaces before/after it.
Add prefix/suffix options for s:get_section.

For output format other than default 1 space padding.
Need this to keep original section_c style:

    let sl.=l:status_color.' %<'.s:get_section(a:winnr, 'c').' '
Remove space-padding works outside of get_section.
Merge pull request #111 from bootleq/blank-section

Do not render spare spaces for empty sections.
move helper functions into update statusline, they're not used anywhere else
move updating external values into extensions
rename window_override_funcrefs to statusline_funcrefs
move syntastic implementation out of core
fix placement of readonly, add support for startify
swap features and rationale in readme
loop differently so it works in 7.2
fix ctrlp statusline getting overwritten via cursormove
add jellybeans theme using new highlight group methods
collapse section when it is empty text (#70).
Add bubblegum theme
Merge pull request #114 from p-m-p/master

Add bubblegum theme
add modified colors to jellybeans
first pass at tomorrow themes (#67, #113)
extract colors from normal if undefined (#67, #113).
tweak tomorrow colors, covers all dark themes pretty well. resolves #67 and #113.