Convert age matcher to use a duration config

[himynameisdave]

Adds the ability to specify ranges for the age matcher. Same as the last-modified matcher, this makes use of the same DurationConfig struct. Maintains support for the previous string matcher (ie: age: 8d), but removes references to it in the documentation and marks it as "deprecated" (so that if we wanted to clean stuff up on the next major version bump, we could remove this).

# Before:
  - age: 7d

# After:
  - age:
    at-least: 7d

Closes #158

[srvaroa]

Thanks for this one! It looks like having both use age in the yaml confuses the parser. With this change it works correctly.

I think it's fine to keep Age deprecated and offer the range with AgeRange to avoid doing contorsions with the parsing. Otherwise looks ok for me, thanks a lot of taking this one.

[himynameisdave]

@srvaroa should we continue to document the Age in the readme?

[srvaroa]

@himynameisdave i think we can remove it unless you see a strong reason not to

[himynameisdave]

yeap I'd prefer to remove!

[himynameisdave]

@srvaroa ready for review / please run the tests!

[srvaroa]

Thanks for the feature!