added support for fieldNameSize

Hi. I added support for fieldNameSize and corresponding test parameters. Kept the 100 byte default, as specified by the documentation. This way if anyone says busboy broke their implementation, well, the intention was always for the default to be 100 bytes. Only put the functionality in place for content-disposition for multi-part data. If this was wrong or I'm missing something, please let me know. Otherwise this should close issue https://github.com/mscdex/busboy/issues/6 for you. Thanks for the library. I'm using it via multer and am trying to harden the security of my submission system.  Oh, and added a .gitignore file. Hope that's okay too. Thanks again for the library.  (p.s. I didn't give this a version bump, though)
Yes, line 79 could be combined with 77. I had a console.log checks writing out characters here to verify I wasn't truncating the wrong fields. Yes, I can drop the curly braces too.