Add snap packaging support

[ltrager]

This adds Snap packaging to jq. The snap can be built by running
make snap

The jq snap can then be published to the Snapcraft store by following the Snap publishing document

[coveralls]

Coverage increased (+0.4%) to 85.752% when pulling 226adc4 on ltrager:master into 0b82185 on stedolan:master.

[coveralls]

Coverage increased (+0.4%) to 85.734% when pulling 2b0bb80 on ltrager:master into 0b82185 on stedolan:master.

[cprov]

I think it should be adjusted to match the suggested building setup:

configflags:
  - --disable-maintainer-mode

[cprov]

@ltrager, I've completely missed your PR and proposed another, #1210, apologies.

I've suggested some changes here for your consideration so it supersedes mine.

Thanks for your work on this.
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[rgrannell1]

Has there been any progress on this issue? I'm hoping to migrate away from .debs where possible