Added custom dimension 1 support to the Event class

[Kimblebrook]

Events can be used to submit optional custom dimension values. I've added an optional custom1 dimension to the Event payload to support this. Since you can have a number of custom dimensions, I am planning to enhance this to use a list, but wanted to get this PR in first. Feedback and suggestions welcome!

PS: I love this library, so thanks so much for making it!

[codecov-io]

Current coverage is 88.53%

Merging #11 into master will decrease coverage by -1.14% as of af6bc3a

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  Stmts          155     157     +2
  Branches        37      38     +1
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  Hit            139     139       
- Partial          8       9     +1
- Missed           8       9     +1

Review entire Coverage Diff as of af6bc3a

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[patrys]

Since it changes the API, I'd rather merge it in its final form to avoid changing the API twice. Would you mind sharing what you'd want the final API to look like? I think a tuple would make more sense than a list since it's the positions that matter, not the order (you always want metric n to represent the same thing).

[Kimblebrook]

That makes a lot of sense, @patrys. I'll have a think about it and also look into the tuple too.