Parse a foo:bar=<valu> kwarg into (prop,valu),off

def parse_cmd_kwarg(text, off=0):
    '''
    Parse a foo:bar=<valu> kwarg into (prop,valu),off
    '''
    _, off = nom(text, off, whites)

    prop, off = nom(text, off, varset)

    _, off = nom(text, off, whites)

    if not nextchar(text, off, '='):
        raise s_exc.BadSyntax(expected='= for kwarg ' + prop, at=off)

    _, off = nom(text, off + 1, whites)

    valu, off = parse_cmd_string(text, off)
    return (prop, valu), off