Smart buttons

I enhanced the actions of the toolbar.

If the cursor is on an empty line, by default buttons will insert the exemple markup. Otherwise their behavior will depend on whether something is selected (for example for the link button), or if the button acts on the whole line (for example for the headers buttons).

When markdown that create a block element is inserted, it will also check it is separated from the surrounding text by 2 line breaks. Fixing issue #6.

I also had to update rangyinputs to the last version to fix some selection problems. And I corrected its file name.

Finally I fixed the focus not comming back to the editor after a click on the toolbar. See issue #5.

(I also wanted to thank you for Meltdown and to let you know that I am  going to use it in a [DoKuWiki](https://www.dokuwiki.org/) plugin: [naokij/dokuwiki-plugin-markdownextra](https://github.com/naokij/dokuwiki-plugin-markdownextra/) See: pull request naokij/dokuwiki-plugin-markdownextra#8)