A way to register parsers

[m1guelpf]

To bypass an edge case I ran into, I had to add a register a custom parser. Unfortunately, there's no other way to do it than overwriting the container registration in the bootstrap.php file.

It'd be nice to have a $jigsaw->registerParser() function for situations like this. This function would ideally be able to

• Register a parser
• Specify the order in which to register a parser (first or last would work)
• Be able to register parsers in other places that accept parsers (like
  the CollectionItemHandler class).

Is this something you'd be interested in having? I can PR it if you want :)