libreddit: Allow header to be included with C++ code

This patch allowed the reddit.h header to be used with C++ code, via the
addition of extern "C" {} around the code if the __cplusplus flag is
set.
Merge pull request #61 from DSMan195276/cplusplus-fix

libreddit: Allow header to be included with C++ code
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
creddit: align index on LinkScreen to at least 2 digits
Merge pull request #64 from olof/topic/link_index_align

creddit: align index on LinkScreen to at least 2 digits