DeckTape cannot add JPG/PNG exports on Windows

[ysb33r]

Currently add screenshot exports with DeckTape fails on Windows.

Consider this line where the command was executed

C:\Path\To\node.exe \
  C:\OtherPath\To\node_modules\decktape\decktape.js \
   --screenshots \
   --screenshots-directory ..\..\..\..\..\..\..\..\..\.. 
   --screenshots-format jpg \
   --screenshots-size 1024x768 \
   reveal \
   file:/C:/Users/appveyor/AppData/Local/Temp/1/junit8625998783273610388/build/docs/asciidocRevealJs/index.html \
  C:\Users\appveyor\AppData\Local\Temp\1\junit8625998783273610388\build\generic\index.pdf

It fails with

Error: ENOENT: no such file or directory, open 'C:\C:\Users\appveyor\AppData\Local\Temp\1\junit8625998783273610388\build\generic\index_1_1024x768.jpg'

Note the double use of C:\C:. This could potentially be an issue with how DeckTape resolves paths that touches the root of a Windows filesystem.

See: astefanutti/decktape#181