Add filepath option as parameter

[mlb6]

I had a use case where I needed to modify the value of options.filepath. So I passed this value as a parameter of gulp-coveralls.

Usage:

gulp.src('/**/lcov.info')
.pipe(coveralls({filepath:"build"}));

[coveralls]

Coverage remained the same when pulling ccea7e67de2d44a7c14add9c7620532918652152 on mlb6:master into 83ed9eda59f4757e2eac85c7f51430e8abf5b9ab on markdalgleish:master.