Headless mode

[cohix]

Atmo should be able to start up in a 'headless' mode wherein it does not build a router based on routes in the Directive, but rather makes each Runnable in a bundle available as an individual endpoint.

This would essentially be a 'FaaS' mode that wouldn't require routes to be explicitly defined.

I envision this being activated with an ATMO_HEADLESS env var.

[cohix]

In this mode, Atmo should be able to dynamically load new Runnables (i.e. the routes should not be static, but accept any function name). This will rely on #41 and possibly #40 in some cases.