Update php and composer commands to allow passing in specific site

[mattstauffer]

Closes #1367.

E.g., to run this command:

valet php -v

When you've linked a site using a name other than the directory name; for example:

cd /tmp
mkdir my-site
cd my-site
valet link different-name-than-folder
valet isolate different-name-than-folder [email protected]

This PR allows you to run this:

valet php --site=different-name-than-folder -v

[scrummitch]

Working locally for me, test output here:

#1367 (comment)