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.