Use vertical-scroll-bar frame parameter.

The use of scroll-bar-width was incorrect and probably the cause of a
bug (cf. Github issue #4).
Maximise window when `writeroom-mode' is started.
Update for new double dash coding convention.
Change layout of `writeroom-global-effects' value.
Update obsolete version string on `writeroom-global-functions'.
Add option to enable a border around the text area.
Update doc string of `writeroom-border-width'.
Update README.md with info on the border option.
Update README.md
Store original global effect values in frame parameters.
Remove superfluous comments.
Update doc string of writeroom--mode-line.