def from_github(user_repo_pair, file='plashfile'):
    "build and use a file (default 'plashfile') from github repo"
    from urllib.request import urlopen
    url = 'https://raw.githubusercontent.com/{}/master/{}'.format(
        user_repo_pair, file)
    with utils.catch_and_die([Exception], debug=url):
        resp = urlopen(url)
    plashstr = resp.read()
    return utils.run_write_read(['plash', 'build', '--eval-stdin'],
                                plashstr).decode().rstrip('\n')
build and use a file (default 'plashfile') from github repo