Extended string interpolation

[Chimaine]

With the addition of simple strings (see #20), string interpolation can be brought a step further, namely supporting expressions.

The new interpolation pattern will use $identifier as it's simple form, and ${expr} as it's complex form.

The only expressions that won't be allowed are table constructors, since those use the same curly braces.

Note that in order to keep backwards compatibility to 2.2, {identifier} will not be removed, but deprecated and scheduled to be removed in 2.4.

[Chimaine]

See #22 for an proposal for alternative table constructor symbols, that would enable their use within complex interpolations.