remove unused "parseOptions" option, and add default options in lexer

[adamkiss]

What I did:

• parseOptions in lib/index.js was unusued, removed it
• added default empty object as an options to lexer (a la reshape/parser)

Why:
Reshape by default calls this function with (input, opts.parseOptions, opts). In none of the tests was parseOptions defined, so error tests kept failing by trying to access undefined.filename when generating errors. Looking at reshape/parser, I realised that lexer has just to explicitly start with empty config if none is given.