Add option to not render blank lines

This PR adds an configuration option to the HTML formatters that can enables or disables rendering blank lines:

const song = (perform your parsing here);
const formatter = new ChordSheetJS.HtmlDivFormatter({ renderBlankLines: false });
const formattedChordSheet = formatter.format(song);

Group lines in paragraphs

As of this version, besides the Song#lines property, the song contents can also be read off the Song#paragraph property, in which the lines are grouped in paragraphs, leaving out the single blank lines.

v2.4.4

Changes:

Fixed bugs

Handling whitespace

The HTML formatters were trying to be smart about whitespace, which resulted in bug #52. The following chord sheet:

[C]Wisper words of wis[G]dom

was accidentally formatted as (note the space inside "wisdom"):

C                   G
Wisper words of wis dom

As of this release (2.4.4), it is formatted as:

C                  G
Wisper words of wisdom

See #52 and #53.