Convert extensions to array

If `extensions` is passed and not already an array, it will convert it into one.

Other changes:
 * added `tape` as a devDependency
 * added `gitignore` for `node_modules/`
 * added a test for the optional `.` prefix for extensions
 * added a test for the new feature above
 * added `sync:` and `async:` prefixes to test names