Unfinished support for aria2 and axel engines
(Hopefully) working implementation of the aria2 and axel engines

Untested, but includes updated documentation
Fix errors in the aria2+axel code.

* ariaII => aria2 in documented options to match code
* Remove predicate in \__download_if_executable_test:n family
* Temporary boolean in \__download_if_executable_test:nTF removed
* Add -L flag to cURL calls
* Fix aria2c calls
* Don't use predicate in \__download_if_auto_test:TF (expandibility issue)
Merge pull request #2 from urdh/aria2+axel

Add support for aria2 and axel engines.