Multiline strings in meta section.

This commit implements multiline strings in the meta section. It is now possible
to do the following:

rule a {
  meta:
    b = """
This is a multiline string literal.

	\x41

Hooray!
"""
  condition:
    true
}

Escape sequences are handled so the sequence "\x41" in the above example comes
out to a single 'A'. Everything else is copied into the rule verbatim.

This is different behavior from regular strings in that things like "\t" are
converted to a single byte (0x09). If you want a tab character in your
multiline string, you must put it in yourself. Same goes for newlines.