Version 0.5.0

• Jupyter magic commands escaped when exported (#29)
• 'endofcell' option for explicit (optional) end-of-cell marker (#31)
• 'active' cell option now supported for .py and .R export (#30)
• Raw cells now preserved when exported to .py or .R (#32)
• Extensions can be prefixed, like .nb.py, (mwouts/nbsrc#5)
• When a file with an extension not associated to 'ipynb' is opened and saved,
  no 'ipynb' file is created (mwouts/nbsrc#5)
• Extensions can now be a sequence of groups. For instance,
  nbrmd_formats="ipynb,nb.py;script.ipynb,py" will create an ipynb file
  when a nb.py is opened (and conversely), and a script.ipynb file when a
  py file is opened (mwouts/nbsrc#5)
• nbsrc script was moved to the nbrmd package.