Add support for backtick to use instead of double-quote at times for string-literal assignment

[merarischroeder]

Solving this issue: https://stackoverflow.com/questions/14480724/escape-double-quotes-in-a-string

When there are a lot of double-quotes to escape within a string, it becomes labour-intensive and also less readable.

I propose that the backtick character can be used exactly the same as a double-quote character, except that escaping of double-quotes won't be necessary.

This is similar to how Javascript lets you use either single-quote or double-quote. It's nice to have both options.

The coder may then choose between backtick or double-quotes accordingly to define strings, and gain better readability when using.

string test = `He said to me, "Hello World". How are you?`;

Another alternative could be the MD way (which I just discovered while trying to make the code-block MD work) - see markdown-it/markdown-it#604. If you 2x-double-quotes to start a string, that's how it ends, then you just use of a 1x-double-quote becomes literal.

string test = ""He said to me, "Hello World". How are you?"";

[HaloFour]

See also: #89