Gerber file starting with newline incorrectly identified as drill file

[mcous]

Ticket taking over for mcous/gerber-parser#23

The file determination routine is pretty naive, basically saying "if I see an * it's a gerber, but if I see a \n first it's a drill". This is a problem because it's perfectly valid for a gerber file to be padded with newlines at the beginning, but gerber-parser will detect a drill file instead.

A file should probably only be called a drill upon the first newline encountered on a non-blank line. Also related, the discussion in mcous/gerber-parser#16 talked about potential problems with the drill/gerber filetype detection (which are no longer potential).

As a workaround, passing the option filetype: 'gerber' will force the parser to parse as a gerber, unless you're trying to use parser.parseSync because of another bug (see mcous/gerber-parser#22)