Added support for a config file

I wanted to have a config file, that I could place among my dotfiles, that allowed me to use dosage, in other computers, with minimum setup. 

To use the config file in the default place("~/.dosagrc"), just call "dosage comic -f"

This config file supports: 
    adult: Instead of using --adult
    path: Instead of using --basepath

comics: A dict with the name of the comic and last downloaded file.
@ will now go to the config file, to know which comics to download
Also, when using --continue, it will check for the last downloaded filename in the config file to stop.

I refrained from changing the documentation, as to give a chance to the current maintainers of this project, to change it to their will.