Commands: Argument-aware tag filters

[Yuffster]

Following #21, command filters should be able to take the previous argument as input.

For example, the command fragment transfer 10µl from <well:start> should be able to return all wells with 10µl of movable liquid, accessible as their string values (['A1:A1', 'Label:A1']), etc.

transfer 10µl from A1:A1 to <well:end> should be able to return all wells with 10µl of unallocated space for liquids.

This would allow for a dynamic user interface for editing Protocols through the direct command syntax.

This can be accomplished as an extension to the filter syntax.

For example...

transfer 10µl from <well:start> would produce...

command_context.filter_tags('well', volume=10)

transfer 10µl of buffer from <well:start> would produce...

command_context.filter_tags('well', volume=10, content='buffer')

An optional flag can set the maximum amount of filtered items to the list.