Parse TemporalAmounts besides Duration

[nrktkt]

The config currently has a handy method getDuration which parses readable strings like "5 minutes" into a java.time.Duration.

It would be nice if there was also a way to parse other TemporalAmounts such as Period ("2 years").

[havocp]

If this supports the same units as duration (maybe adds months?) I think it would be pretty straightforward to add a getPeriod method, and I don't see a reason not to do so. The approach I'd take would be to find everywhere that has duration now and add this along the same pattern. If there are any new units added, HOCON.md would need to be updated to reflect that.
Thanks!

[nrktkt]

@havocp it would add days, weeks, months, and years. If you have any pointers for places to start I can look into throwing together a PR.

[havocp]

I'd essentially look at getDuration and copy that; it should have unit tests to copy also. This is a fairly "surface level" feature of the library (conversion from string or number to duration/period is late-binding when someone calls getDuration) so you wouldn't need to touch the parser or other lower level stuff afaik.

[nrktkt]

@havocp is there a reason the compiler targets java 6 source even though java 8 classes like Duration are already being used? (I was looking to use default methods in interfaces and string switches)

[havocp]

I don't remember why that is; suggest digging up the PR discussion when we switched to java 8.

[2m]

Implemented in #478