add host option runGadget

[xiaodaigh]

I sometimes specify host for runGadget apps to allow the specification of host. This is useful for apps lie equisse that uses the runGadget instead of runApp

[alandipert]

Hi, thank you for the contribution. I agree it makes sense for runGadget to have a host parameter too.

However, I couldn't find a record of an RStudio contributor agreement corresponding to your GitHub username. We require all contributors to complete and submit one before we accept their contribution.

If you've submitted one already but it's associated with your name or email address and not your GitHub username, please send me an email at [email protected] and we can figure it out.

Otherwise, please review CONTRIBUTING.md for instructions.

Lastly, this PR is missing an update to NEWS.md.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[schloerke]

@xiaodaigh I can not manually change the base merge branch. 😞

Do you mind resubmitting the PR? Sorry for the hassle