add triple-quote string literals, e.g., """abc"""
strip leading whitespace from triple-quoted strings (closes #70)

For example,

    s = """
        a
         b
        """

is now equivalent to "a\n b".
use the last line, not first, to remove triple-quote string whitespace
tweak triple quoted string indenting algorithm

Restores the trailing newline when the terminator occupies its own line.
Otherwise, instead of using the last line to set the deindent level,
scan for the longest common prefix.

Also, handle tab/space mixing.
Merge pull request #2305 from JuliaLang/mn/triplequote

add triple-quote string literals, e.g., """abc"""