Custom extension for format

[passcod]

Tangentially related to #37.

Currently the Tgz format assumes the .tgz extension, but it might instead be .tar.gz.

In the current state, one may simply not use the { format } specifier for the URL and add the correct extension there directly. However, with some solution to #37 there's going to be a need for this mapping!

[ryankurte]

hmm yeah that's an interesting one, there's also a hack to add .exe for windows binaries, it'd be nice if we could expose both of those in an obvious manner somehow...

i had wondered about a map of target-specific overrides under the configuration so, for example, you would set your path up as a normal template but for windows have a [package.metadata.binstall.target.x86_64-pc-windows-gnu] with a format = "zip", do you think that'd cover this / #37?

[passcod]

That would work 👍