creddit: Add 'alloc_sprintf' function.

This patch adds a 'alloc_sprintf' function, which take the same
arguments as sprintf and returns a buffer allocated to the exact size
neeed by that sprintf. The buffer needs to be freed by using free().
creddit: Add opening links in the default browser using 'o'

This patch adds functionality to the link-screen. It opens the url of
the current selected link in the default browser using xdg-open.
Merge pull request #63 from DSMan195276/open-link

creddit: Add keypress to open a link in the default browser