Add options for highlight

- Add options for highlight by color args and group.
- Add local variables that are stacked color args.
- Added options are set up by using local variables or "".
Fix highlight implementation

Re-define color highlight when some line is marked, because some
 colorschemes clear highlight definition AFTER plugins are loaded.

Fix parameter of sign definition in DefineSign() as below.
 'texthl' => 'linehl'
Use stacked color args

Use color args that stacked local variable when g:showmarks_hlargs_*
 options are set up with default.
Change timing of re-defining highlight

Invoke DefineHighlight() in ShowMarks().
Support highlight marks on the same line

Refer to options g:showmarks_hlargs_multi, g:showmarks_hlgroup_multi
 when marks are placed on the same line.
Remove unnecessary space
Update document

- Add explnations for highlight options.
- Fix highlight section.
Merge branch 'add-options' into develop
Bump version number to 2.3
Merge branch 'release-2.3'