Fix failing shortcode tests on Travis

Some newly added shortcode tests compared maps in assertions.

This failed on Travis, as iteration order isn't guaranteed for maps since Go 1.

This commit fixes that by do a sort of the keys in the shortcode String() function.
Update shortcodes.md

A typo resulting in a broken link.
Fix broken convert

The front matter convert-feature didn't work. It placed converted content in the wrong directory.

This commit fixes this by doing the smallest and safest change possible; the path logic here should maybe
be revisited and generalized.

Fixes #643
Update shortcode doc for 0.13-DEV

Update of the doc to reflect the shortcode  implemention in HEAD.