Add support for custom site domains to valet php and valet composer

[aj-adl]

Fixes #1272

Adds support for passing in a custom sitename via the flag --site=sitename

• Could be more flexible, at this point the '=' is mandatory
• Command help updated, arguments added etc
• Code is repeated in two place, I thought it more likely that potential flags / checks diverge over time and didn't want to prematurely abstract

Potential issues

• someone calling a php command that requires a "--site" flag has to encapsulate the whole command as a string to pass it in - e.g valet php 'cms_cli do:thing --site=radsite.test' --site=radsite.test

Other considerations

• It would be better if the isolate function wasn't so tied to dir names - keeping a config file of isolated sites vs just parsing nginx configs seems better to me in the long run but is a larger and more serious refactor of valet.

[mattstauffer]

@aj-adl It's been a while since I looked at the isolation code. Could you give me an example of a configuration in which this would be necessary?

Thanks!