Conditional extension invocation

[philbooth]

Currently a single boomcatch instance executes with a single extension pipeline configuration. However it is unlikely that anyone using multiple boomerang plugins wants to handle all of the data in the same way.

For example, you might want to map navigation timing data as statsd timers and resource timing data as a waterfall chart.

This would require a way to conditionally specify multiple extension configurations.

Doing that on the command line would likely be a bit of a mess, trying to define some arbitrary DSL that handles all use cases perfectly. A better approach could be to expose a meta-extension, a JavaScript extension point that lets you invoke the other extensions however you like.

[philbooth]

As mentioned in #56, this could be handled during the filter step.