feat: Remove source/posts/layouts flags
feat: Remove source/posts/layouts flags

The main use cases for flags that mirror the config file
- One-off overriding.  The local setup might be different than the
  deployed.  This is more for cases like `dest`.  For the flags being
  removed, I cannot see why someone would need to override these config
  settings.
- No config file.  This just seems error prone.  I don't think this is a
  use case we are aiming to support.

So why remove?
- Less to test
- Less semantics to worry about defining.  Should these be relative to
  the config file, CWD, etc?

This is a step towards #284

BREAKING CHANGE: --source, --posts, --layouts flags have been removed.

cobalt-org/cobalt.rs,cobalt-org/cobalt.rs,cobalt-org/cobalt.rs,cobalt-org/cobalt.rs