Add filtering support in flux-commands.md

[fsteeg]

This is part of metafacture/metafacture.github.io#10

Add filtering support in flux-commands.md, e.g. generate filter 'buttons' for each type as input and output, e.g. ->StreamReceiver for all decoders, StreamReceiver-> for all encoders, etc.

This probably requires some HTML format. That format could be optionally generated in HelpPrinter.java like the .md. It might work to actually add some HTML to the Markdown, like <details>/<summary> for the examples in collectors.md.