getBasename function supports filenames with multiple '.' in extension

Using join intend of toString allows this package to support src file names with multiple '.' (periods) in the filename.

With config
  template: "templates/*.hbs"
  output: "www/*"

and template dir containing:
   config.xml.hbs
   index.html.hbs

Before PR:
  wrote files: config,xml and index,html (notice the bad comma)

After PR:
    wrote files: config.xml and index.html (looks good!)

Basename will always be an array - using join('.') here instead toString() is fine.