(GH-551) adding unset to config commands

[zhao-xu]

Add unset to config commands so proxy can be switched on/off in scripts.

For example:

choco config set proxy http://localhost:8118
choco install package_requires_proxy_to_install
choco config unset proxy
choco install package_do_not_need_proxy_can_speed_up

Closes #551

[ferventcoder]

This is great as far as the code goes! I thought I had already created an issue for this - but apparently I never got to actually creating the issue. Would you read over https://github.com/chocolatey/choco/blob/master/CONTRIBUTING.md and create the issue and then update your commit message as mentioned in the document? Then we can move forward with this!

[zhao-xu]

Sorry, I forgot that. Document's too long, didn't read them all :-p
I went throught the issue list last night and didn't find any related so I created a new one #551
Commit message has been updated to metion the issue number.

I'm not sure whether I should create a new PR or this one will be updated automaticlly?

[ferventcoder]

Document's too long, didn't read them all :-p

Better to document explicit expectations rather than leave it up for folks to interpret themselves. :)