parameter for corpusdir location in plos_corpus.py

[eseiver]

In preparation for pip install allofplos, there needs to be a way for the user to specify the path to corpusdir. Right now, the xml folder is created within the repository, and most people won't want to store datasets with executables. Some ideas:

• default corpusdir location that would work across platforms, or change depending on OS (~/Documents/ ?)
• request and store user input for the path to corpusdir, default to cwd/pwd

[eseiver]

This was partially addressed in #70 with the PLOS_CORPUS environmental variable, but it doesn't store permanently since it's using os.environ. Also worth noting that the default corpusdir location is no big deal for a cloned repo, but for the pip install it will wipe any directory in location whenever the package is upgraded.