Add support for block params for custom HTML tag syntax for components

Initial attempt for https://github.com/tildeio/htmlbars/issues/117.  There was a fair bit of minor copy-paste here and there, but I wasn't sure whether it's ok or warrants some refactoring to reduce duplication, so I figured showing it first is better.  I also wasn't sure whether I need to add a test to `hydration_compiler_test.js` (and I haven't quite learned the opcodes DSL yet).  And, of course, I hope I chose the right place to do the main processing (the call to `parseComponentBlockParams`).