Support 'file' option for reporters

[jreichenberg]

Many istanbul reporters (ex. cobertura) take 'file' as an option in addition to 'dir'. gulp-istanbul currently only passes 'dir' through in the reporter opts via:

reportOpts: { dir: opts.dir || defaultDir }

if this is changed to

reportOpts: { dir: opts.dir || defaultDir, file: opts.file }

multiple runs of gulp-istanbul can create individual report files in the same dir without overwriting.

[SBoudrias]

Send a PR ;)