"Comment" formatting tag

[jm-welch]

Overview

Formatting tag to allow adding non-lyric text to song output with special formatting.

Requirement

• Requires text
• Supports both long and short forms: {comment: Comment} == {c: Comment}
• Adds comment to output at the tag's location with "comment" formatting (tbd)

[jm-welch]

This won't really be needed until formatted (non-plain-text) output is supported, but is part of the Chordii spec, so I'm including it up front.