Make `json-mode-beautify` work without a mark.

It is possible to not have a mark (for example, just after opening a
file) and hence, `json-mode-beautify` didn't work in this case. We don't
take a region as a parameter but we use the current active region
instead.
Merge pull request #17 from vincentbernat/fix/work-without-mark

Make `json-mode-beautify` work without a mark.