Allow for custom host with pubsub

[mkamioner]

When using the emulator, I don't want to be restricted to using the environment variable of PUBSUB_EMULATOR_HOST - Especially when I have certain topics in the cloud and other in an emulator.

This will allow the programmer to decide what host to use easily

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[mkamioner]

I signed it!

[googlebot]

CLAs look good, thanks!

[coveralls]

Coverage remained the same at 100.0% when pulling c519c37 on mkamioner:master into 13ec5fe on GoogleCloudPlatform:master.

[stephenplusplus]

Thank you for this! We support this in the Datastore API, but haven't brought it over here yet, so your help is appreciated!

To match what's going on in Datastore, could you rename host to apiEndpoint? You can see (and please add) the JSDocs description here: https://github.com/GoogleCloudPlatform/google-cloud-node/blob/13ec5fef0da92b74e7adf7a2479f8c1636be72d4/packages/datastore/src/index.js#L61

And if you're up for it, we could use some tests as well. You can probably get by with copying what we have in Datastore: https://github.com/GoogleCloudPlatform/google-cloud-node/blob/13ec5fef0da92b74e7adf7a2479f8c1636be72d4/packages/datastore/test/index.js#L363 -- if you'd rather have me do it, no problem, just let me know.

Thanks again!

[mkamioner]

@stephenplusplus - Happily! I will do it now!

Happy to contribute :-)

[coveralls]

Coverage remained the same at 100.0% when pulling 44f70df on mkamioner:master into 13ec5fe on GoogleCloudPlatform:master.

[stephenplusplus]

It's perfect, thank you very much again!

[mkamioner]

Honor and a pleasure :-)