Make devtools:documentation --directories smarter

[Hywan]

Related to hoaproject/Central#19.

Currently, the default value for --directories is nothing. So if one would like to generate the documentation for the current library, we need to specify --directories . which is annoying and it does not help when writing documentation.

The default behavior must be smarter by having the current directory by default, so that we no longer need to write --directories .. Any new value could be added or overwrite the default value. I don't have any opinion about it, expect it would be better to overwrite the default value to prevent future BC breaks.

[Hywan]

Fixed by #33.