Interpolated Heredoc
Multiple line double quoted strings

When adding heredoc I realized I hadn't added support for multiple lines
for normal double quotes.
Add multiline support for single quoted strings
Rename DOUBLE_QUOTED_STRING to INTERPOLATED_STRING

Matches convention used for heredoc and Color Settings better.
Rename SINGLE_QUOTED_STRING to STRING

Matches HEREDOC vs INTERPOLATED_HEREDOC pattern.
Add interpolated heredoc to Color Settings example

Add triple double quotes to set of tokens highlighted as interpolated
string.
Use yypushback to eliminate duplication between YYINITIAL and BODY

Instead of calling yybegin(BODY) and returning the same thing as BODY,
push back the text and begin BODY so all the token specific actions can
remain unduplicated in BODY.
Highlight escaped single quotes in strings
Separate string and interpolated string fragments

This way interpolated fragments and double quotes can be highlighted as
'Interpolated String' and fragments and single quote can be highlighted
as 'String'.
Highlight single quotes and string fragment as String