Add BuildWithCustomFormat trait

[sile]

Closes #22, #55

[coveralls]

Pull Request Test Coverage Report for Build 7675457341

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A quick fix for this PR: rebase it; your next report should be accurate.

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 25 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.7%) to 47.541%

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Files with Coverage Reduction	New Missed Lines	%
../../../.cargo/registry/src/index.crates.io-6f17d22bba15001f/crossbeam-channel-0.5.11/src/flavors/array.rs	2	82.29%
../../../.cargo/registry/src/index.crates.io-6f17d22bba15001f/crossbeam-channel-0.5.11/src/select.rs	2	76.19%
../../../.cargo/registry/src/index.crates.io-6f17d22bba15001f/crossbeam-channel-0.5.11/src/context.rs	8	64.15%
../../../.cargo/registry/src/index.crates.io-6f17d22bba15001f/crossbeam-channel-0.5.11/src/waker.rs	13	40.91%

Totals
Change from base Build 7647967786:	-0.7%
Covered Lines:	1605
Relevant Lines:	3376

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💛 - Coveralls