Additional support for string literals

[MattEdwardsWaggleBee]

This does a combination of options from #194

• Allows backticks as a quote type
• Allows custom quote types (might not be worth it, I can delete those parts)
• Makes ReadQuotedString public so developers can inherit and create custom string literals

[sebastienros]

Note that eventually it should expose more useful sets of expressions like CSharpString, JavaScripString, SQLString ... since they actually have different logic for escaping and that's probably what we want when parsing quoted text. For instance single quoted strings in JS use \' to escape the single quote, while in yaml and sql it's ''. Could also handle multi-line parsing, like the """ in c# or ``---` in JS.

Probably in a custom Parsers sections or extensions with predefined schemes. I think Irony has such things.

[MattEdwardsWaggleBee]

I'll give that some thought. I know @bfarmer67 has been considering multiple line strings and comment blocks too.