fix autoselection of theme for certain startup configs.
allow specific whitespace checks to be enabled/disabled.
undo use of inverse; caused problems with empty sections.
Add option to show only non-zero hunks
Prior checking user defined theme
fix color switching again. vimrc setting should override.
support x,y,z theming. resolves #159.
Merge branch 'use_defined_theme'

Conflicts:
	plugin/airline.vim
Use extension local variables, and expose hunk_symbols to user
Merge pull request #158 from BenBergman/only-non-zero-hunks

Add option to show only non-zero hunks
reduce pollution of global scope variables
deprecate all g:airline_enabled_* variables.
deprecate airline_detect_whitespace
simplify, no globals unless necessary.
readonly mark is now a member of extensions
remove g:airline_detect_readonly
Merge pull request #161 from itchyny/readonly_symbol

readonly mark is now a member of extensions
deprecate airline_readonly_symbol. move into gutter by default, so it does not conflict with bufferline logic.
hunks: Display the hunks summary only if Git Gutter is enabled
allow warning section to be overwritten (#136).