Refactor v1 release filters

[colmsnowplow]

In reviewing the v1 PR, there were some issues with filtering which were difficult to map because of how the code was structured. This issue documents the changes made in order to resolve that issue, and fixup some of the issues in the filtering feature which can be found in this PR.

Commits referenced below will be squashed and rebased - but keeping the references here in order to refer to the specific changes in turn.

• Code structure factored to be more testable and to separate logic appropriately: 5dcaaa4

This also changes the API for unstruct filters here to make the checks on event name and version explicit on configuration.

• Factor out compiling regexes so that this happens only once on startup: b2c0d68

• When no value is returned, the filter always evaluates to false, and when a nil value is returned we don't match against nil, but against the string <nil>: e643a52

• Use Compile() instead of MustCompile() to avoid panics: 61dbba8

• Issue with the analytics sdk's API (Return []interface{} from GetContextValue snowplow-golang-analytics-sdk#36) leads to error when value doesn't exist, rather than simply matching nil. db5841e

• Incorrect tests: 5dcaaa4 and b77d244

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Additionaly, the following are things that I think we should improve, but shouldn't block v1 release for. Documenting them here in the name of completeness:

• We should avoid negative lookaheads: Change the filter API to avoid negative lookahead #188
• Config improvements: Change the filter API to avoid negative lookahead #188
• Simplify unit tests: Simplify unit tests #190
• Fail earlier on misconfiguration of base filter: Base filters should fail on startup when the field provided is not an atomic field #191