pkg-stats: allow to sort by column

pkg-stats: allow to sort by column

With 2000+ packages it's not trivial to identify i.e.:
- all packages that don't have a hash file;
- all packages that have patches;
- all packages that have code style warnings;

User experience can be improved by dynamically sorting the resulting
table.

There is an open-source solution that does that in the client-side and
requires minimal changes to our script: sorttable.js.  The script is
MIT licensed as stated in its website.

Also add a hint to the user that the table can be sorted.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>