Update to cmdargs-0.10.20

0.10.20, released 2018-01-22
    #54, use the getopt data types from base
0.10.19, released 2018-01-01
    #47, ensure Semigroup instance on all GHC versions
0.10.18, released 2017-09-24
    #47, GHC 8.4 compatibility
0.10.17, released 2017-03-31
    Add processValueIO for more controlled error messages
    #529, don't include the stack trace in processValue
0.10.16, released 2017-03-22
    Minor improvement to error messages
0.10.15, released 2017-03-06
    #43, GHC 8.2 compatibility
0.10.14, released 2016-02-16
    #39, ensure correct line breaks in HTML help output
    #18, preserve manual \n in help messages
    #25, reformat the README

(pho)