show readme and changelog as markdown

the user is opening the files for reading not for editing, therefore we
should present the files as rendered markdown
adding check for markdown-preview being active

if not active just open the markdown file without rendering
lets make markdown-preview do the rendering

this is depending on markdown-preview version
[0.58.0](https://github.com/atom/markdown-preview/releases/tag/v0.58.0)
no need for roaster as a dependency anymore
change to one-liner
using interpolation and encodeURI for path
whole URI should be encoded just to be safe
no need for encodeURI here
Merge pull request #99 from tcarlsen/master

show readme and changelog as markdown