Pass rev.manifest path, base, cwd options to vinyl

Add an optional `path` argument, all of these are now valid ways to call rev.manifest:

    rev.manifest('manifest.json')
    rev.manifest('manifest.json', {merge: true})
    rev.manifest({
      path: 'manifest.json',
      base: process.cwd() + 'dist'
    })
    rev.manifest('manifest.json', {
      base: process.cwd() + 'build/dist',
      cwd:  process.cwd() + 'build'
    })
Clean up readme a bit, address feedback.