Multiline strings in meta section.

[wxsBSD]

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.

[wxsBSD]

Going to close this out and implement it in yara-x instead.