Make mmmagic an optionalDependency

[thusoy]

The move from extension-based detectionto sniffing MIME-types in 1.1.0 with mmmagic causes installs to fail on platforms without a C compiler, such as Windows without Visual Studio.

Would be great if you could put mmmagic into optionalDependencies and detect on runtime whether it's available or not, and then fall back to the old extension-based detection, so that the plugin is installable on all platforms.

[mihhail-lapushkin]

Done.

[thusoy]

Great, thanks for the quick response, with the missing line from #17 it works great!