code formatting

No (real) functional change. The regular expression string "Cabal
source..." was reduced to "source...", but it works just as before.
coding style: use double brackets

This is to be merely consistent with the rest of the script. We should
only deviate from convention if there is no other way to code it.
massive rewrite: make everything as parallel as possible

We do bulk updates with multiple arguments to `cblrepo add`, instead of
calling it anew for every single argument. Also, we use `aria2c` (Arch
Linux package: `aria2`) to download cabal files in parallel.
README: update