Allow variables inside double quoted strings.

Example: "$foo"
Implement AST for variables inside an expandable string.

Simple variables inside a double quoted string are added to the
expandable string's nested expressions.
Implement script extent for parse tree nodes.
Add script extent for nested expressions inside expandable strings.
Expand variables inside double quoted strings.

"a = $a"
Update authors.
Merge branch 'master' into expandable-strings

Conflicts:
	AUTHORS.md
Merge pull request #177 from mrward/expandable-strings

Expand variables inside double quoted strings