add packr support

[nkonev]

Along with go-bindata it' s will be cool to add packr.

[dhui]

Thanks for your efforts, but the current plan is to add support for the http.FileSystem interface. Once that's done, support for packr sources should be trivial.

For more info, see this issue and these PRs: original, follow up.

If you'd like support for packr to be merged in sooner, feel free to contribute to either of those PRs and open a subsequent PR for packr support.

[coveralls]

Pull Request Test Coverage Report for Build 301

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 59.188%

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Totals
Change from base Build 298:	0.0%
Covered Lines:	773
Relevant Lines:	1306

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💛 - Coveralls