Add gerbers zip file format

[vikulin]

Add gerbers zip file format for gerbers parameter.

Example:

gerbers: releases/download/v0.1.6.1/open-gamma-kit-gerber-files-v0.1.6.1.zip

Gerber and drill files inside the archive:

.
└── open-gamma-kit-gerber-files-v0.1.6.1.zip
    ├── gamma-spectrometer.drl
    ├── gamma-spectrometer-B_Cu.gbl
    ├── gamma-spectrometer-B_Mask.gbs
    ├── gamma-spectrometer-B_Silkscreen.gbo
    ├── gamma-spectrometer-Edge_Cuts.gm1
    ├── gamma-spectrometer-F_Cu.gtl
    ├── gamma-spectrometer-F_Mask.gts
    ├── gamma-spectrometer-F_Silkscreen.gto
    ├── gamma-spectrometer-job.gbrjob

[kasbah]

Thanks @vikulin for making the issue. Maybe we should use the full URL, so not Github specific, i.e.

gerbers: https://github.com/vikulin/OpenGammaKit/releases/download/v0.1.6.1/open-gamma-kit-gerber-files-v0.1.6.1.zip

[kasbah]

Similarly we could support URLs for BOMs etc.

[vikulin]

@kasbah I thought gerbers parameter supports base URL adding it to the relative repo folder path.

[vikulin]

My proposal is to add an optional base_url parameter with <git_repo_path> default value. But it sounds like a subject for another change and it doesn't directly relate to the zip format change.