ability to set "watch=true|false" (and other CLI params?) from Tiltfile

A user should be able to specify the value of --watch=true|false (and possibly other CLI params) from the Tiltfile, in addition to being able to set them by passing them to tilt up.

Specific use case: @kollender wants to avoid wrapping Tilt in a shell script, but wants access to two different modes specified by args: tilt -- --foo=[bar|baz]. Depending on the value of the flag, a different chunk of Tiltfile code will be executed. if baz mode must always run with --watch=false, ideally this could be set in the tiltfile:

if foo == 'bar':
    < bar workflow >
elif foo == 'baz':
    tilt_options(watch=false)
    < baz workflow >

to avoid the user having to remember extra details about the invocation (i.e. tilt up --watch=false -- --foo=baz)

Start by reading how tilt up CLI arguments and Tiltfile evaluation are currently handled. Trace the existing --watch and passthrough -- --foo=... behavior, then define how Tiltfile-set options should interact with command-line values; done means the requested mode can select its workflow without an extra wrapper script and the behavior is covered by tests.