makes cli commands available on windows machines from site root

I wanted to test the command line package manager and updates locally but the bin/gpm commands won't work on windows command line.

The 2 bat files let you run gpm and grav commands from the root of the site using a windows command prompt by dropping the path from the command

    >cd /root/of/site
    C:\root\of\site>gpm version
    You are running Grav v0.9.6