Improve editor support for virtual environments

[RobinDaugherty]

• [x] Add support for BETTER_ERRORS_EDITOR env var, which is used instead of EDITOR if present.
• [x] Add %{file_unencoded} replacement to editor formatting string, so when a URL needs to be unencoded, it doesn't require a Proc.
• [x] Add support for BETTER_ERRORS_EDITOR_URL env var, which allows the editor to be fully customized without modifying code.
• [x] Add support for BETTER_ERRORS_VIRTUAL_PATH and BETTER_ERRORS_HOST_PATH which, used together, cause the virtual path to be stripped from file paths, and the host path to be prefixed.

This refactors editor detection out of the BetterErrors class into a new Editor class, while preserving the call pattern on BetterErrors.editor=.

[coveralls]

Pull Request Test Coverage Report for Build 346260966

• 55 of 55 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 96.838%

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Totals
Change from base Build 346133107:	0.5%
Covered Lines:	490
Relevant Lines:	506

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💛 - Coveralls