Option to restrict webserver to localhost

[passw0rd123]

This should fix #418

• Adds a localhostonly option to the config
• Ability to choose between listening on all interfaces or 127.0.0.1 (v4 only for now)
• Default is preserved for backward compatibility (listen on all interfaces)

Code is untested for now. So please have a look at the changes.

I chose this boolean option (all/localhost) to avoid the need of validating the config input as a correct ipv4/v6 address or empty string(->all interfaces). This might lead to more complexity and maybe dependencies which I like to avoid.

[coveralls]

Pull Request Test Coverage Report for Build 20635506127

Details

• 0 of 7 (0.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 25.671%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
app/config.go	0	1	0.0%
web/serve.go	0	6	0.0%

Totals
Change from base Build 17698624320:	-0.04%
Covered Lines:	909
Relevant Lines:	3541

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💛 - Coveralls

[BuckarooBanzay]

lgtm, thanks 👍