Add ability to create a line of comment characters

Add functionality to insert a line from comment characters and to toggle
back to a blank line.

This requires the following modifications:

Simple function to insert the line of comment characters.

Modify UncommentNormalLine() to detect a line of pure comment characters
and remove all of them (i.e. toggle).

Create default mapping (<Leader>c_)

Add option NERDNumCommentCharsHR to specify the width of the line made
from comment characters.

Update help documentation for new functionality, default key mapping,
and new option.