Feature: Add support for body templates

[stephen-soltesz]

This completes changes started in https://github.com/m-lab/alertmanager-github-receiver/pull/54

The change adds support for specifying a custom alert message template file.

Example invocation would include the flag: -alert-template-file=alertfile.md

Default behavior is unchanged.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 183

• 19 of 19 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 175:	0.0%
Covered Lines:	413
Relevant Lines:	413

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💛 - Coveralls