Command line default values are wrong (or wrongly documented)

[cblegare]

Some argument defined in conf.py will not get used.

for instance :

  -d DOCTREES   path for the cached environment and doctree files, default
                .doctrees when `ablog_doctrees` is not set in conf.py

the arg_doctrees decorator will always set a value to the doctrees parameter of ablog_build. This means that the conf.py will never get used in the following code:

 doctrees = (doctrees or # doctrees always evaluates to True here
        os.path.join(confdir, getattr(conf, 'ablog_doctrees', '.doctrees')))

unless we explicitly use something like

ablog build -d ""