Allow a "read only" mode where `push_settings` are optional

Basically make the configuration lazily load the various parts for pushing / pulling

This would enable one to make a r/o all-repos

I guess this could just be implemented as a builtin noop push and then:

    "push": "all_repos.push.noop",
    "push_settings": {}

[asottile]

all the tools are meant to be composed, in this case you can use something like:

all-repos-manual ... --repos $(all-repos-list-repos)
all-repos-manual ... --repos $(all-repos-grep --repos ...)
all-repos-manual ... --repos $(all-repos-find-files --repos ...)

[sloria]

Makes sense. Works for me.