support Linux

[catdad]

Currently, sharp has issues running in Electron on Linux... it's probably my fault. Investigate and add support for Linux.

See #104 (comment) for more details.

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• test on Linux
• build on Linux
• deploy linux package - see create Linux snap package #210

[catdad]

The sharp issue occurred in Ubuntu 14.04 and 16.04, which were used in travis and on my development VM, respectively. It turns out that the app runs on Ubuntu 18.04 just fine. Considering that this is an end-user application and not a server application, I am okay with that.

[catdad]

See #210 for Linux package creation