Add enumerate feature to convert operator

[van-fs]

This PR addresses a few issues seen with Tealium users.

• The sample rule needs frequent updates to convert strings to numbers
• order_ information was not included in the original sample
• The convert operator could do a better job of intelligently converting to numbers

An enumerate option was added to convert. The intent is that if specified, convert will try to automatically convert strings to numbers. There was a fair amount of cleanup needed in convert, and I broke the handler into stages:

• Enumerate any strings if desired
• Convert any additional properties specifically
• Reduce any single item arrays to a single value if needed

Two test utility methods expectValid and expectInvalid were added to make the validate tests more legible. (We should refactor other tests later).

[TrevorFSmith]

LGTM.

I left a comment asking whether the operator should error or warn when incorrectly configured, but that's nit that you can ignore or address at will.

[TrevorFSmith]

Are there conflicting combinations of properties that we should error or warn about?

[van-fs]

I think I have them all accounted for. The one gray area is probably force with enumerate. I'll add a card to track.

[van-fs]

Validation of rules results in throwing Error, which in the Observer gets handled as logger.error. I've started to use warn more so as an indicator that something unexpected occurred but a total failure did not occur. For example, failing to register a rule is an error. Being unable to convert a property is a warning because as messy as data layers are, it's possible to convert successfully on one page but incorrectly on another. I think it's helpful to dial back this level of feedback so that errors are surfacing more widespread, total failures while warnings could be sporadic or partial failures.