'Loads a configuration from an environment variable pointing to
a configuration file.  This is basically just a shortcut with nicer
error messages for this line of code::
app.config.from_pyfile(os.environ[\'YOURAPPLICATION_SETTINGS\'])
:param variable_name: name of the environment variable
:param silent: set to ``True`` if you want silent failure for missing
files.
:return: bool. ``True`` if able to load config, ``False`` otherwise.'

def from_envvar(self, variable_name, silent=False):

    rv = os.environ.get(variable_name)
    if (not rv):
        if silent:
            return False
        raise RuntimeError(('The    environment    variable    %r    is    not    set    and    as    such    configuration    could    not    be    loaded.        Set    this    variable    and    make    it    point    to    a    configuration    file' % variable_name))
    return self.from_pyfile(rv, silent=silent)
