Make extensions extensible

[Stockless]

In the current implementation, extensions are being handled by using a long if/elseifchain, which is not very extensible, since new extensions requires modifying this block.
As @cncastillo mentioned in PR #558 , it would be good to replace this handling with multiple dispatch, allowing the users to define their own extensions without having to modify this code.

[cncastillo]

Please give more details.