Add support for subscribing/unsubscribing to/from webhook events

[nicklaw5]

This adds a new endpoint for subscribing/unsubscribing to Twitch webhook events. It also includes some convenience methods for identifying individual webhook events when received from Twitch.

TODO:

• Add docs

[coveralls]

Pull Request Test Coverage Report for Build 113

• 88 of 99 (88.89%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 90.985%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
webhooks.go	65	69	94.2%
helix.go	23	30	76.67%

Files with Coverage Reduction	New Missed Lines	%
helix.go	1	90.98%

Totals
Change from base Build 110:	-0.2%
Covered Lines:	656
Relevant Lines:	721

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 113

• 88 of 99 (88.89%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 90.985%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
webhooks.go	65	69	94.2%
helix.go	23	30	76.67%

Files with Coverage Reduction	New Missed Lines	%
helix.go	1	90.98%

Totals
Change from base Build 110:	-0.2%
Covered Lines:	656
Relevant Lines:	721

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 113

• 88 of 99 (88.89%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 90.985%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
webhooks.go	65	69	94.2%
helix.go	23	30	76.67%

Files with Coverage Reduction	New Missed Lines	%
helix.go	1	90.98%

Totals
Change from base Build 110:	-0.2%
Covered Lines:	656
Relevant Lines:	721

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 113

• 88 of 99 (88.89%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 90.985%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
webhooks.go	65	69	94.2%
helix.go	23	30	76.67%

Files with Coverage Reduction	New Missed Lines	%
helix.go	1	90.98%

Totals
Change from base Build 110:	-0.2%
Covered Lines:	656
Relevant Lines:	721

---

💛 - Coveralls

[nicklaw5]

Merging this without documentation, simply because I believe I may want to change the API a little bit after doing some real-world testing.