force require scales, flexible geom

* It seems that `scales` does not always load properly. This fix should avoid the issue.
* The function now accepts any geom setting. Two examples show how this works and illustrate other options.
* Little things here and there.
documentation of new possible arguments
Tidy argument call
Merge branch 'master' of https://github.com/briatte/ngramr into briatte-master

Fix Conflicts:
 * Allow arbitrary geoms
 * ... passes additional parameters to ngram not geom
 * geom_options allows additional parameters to be passed to geom