Optimized existing code

* Got rid of s:LowerMarksList() and s:UpperMarksList(). All list generation now
  happens in s:MarksList()
* ToggleMark() and ToggleMarker() functions have been simplified and accept
  only a single argument viz. the mark/marker to be toggled and acts on the
  current line only.
* Mapping method has been changed. Instead of creating separate maps for 'ma',
  'mb' etc. only a single mapping is created for 'm' which accepts further input
* Functionality of RemoveMarker() has been moved to PurgeMarkers() and
  RemoveMarker() has been removed.
* Check for unique maps via maparg() is no longer performed. Instead mappings
  are appended if any pre-existing maps are found
* Because of above, signature mappings can no longer be cleared. Thus, toggling
  signature now only hides the signs and doesn't clear the maps
* Provided option g:SignaturePrioritizeMarks to control if marks or markers
  should be displayed when both are present on the line
Merge branch 'dev_v2'
Added an option to force sign refresh