add options for doc extractor

[spneto]

When extracting doc files larger then default stdout max buffer size (200KB), I got the following error:
Error: antiword read of file named [[ myfile.doc ]] failed: Error: stdout maxBuffer exceeded.

So I just passed options.exec to exec. So I'm able to configure maxBuffer, like:

textract.fromUrl('http://www.aaa.com/mylargedoc.doc', 
	{exec: {maxBuffer: 400 * 1024}},
	function( error, text ) {});

[dbashford]

Since these changes are in another PR, can you remove them from this one?

[dbashford]

This has the octet stream stuff in it (spelled incorrectly), so going to close and make the small exec change myself.