Make paths more flexible

[kalinchernev]

That's how #31 would be fixed.
Summary of changes:

• upgrades of the core and its deps solves GraphQLError: Cannot read property 'id' of undefined errors
• the additional check for window presence is documented - with it, there are no issues during build time when netlify global is not in the window
• path is passed to the context in order to match user-provided values and what the app will show to the user and in end build bundle.

I've left one example of a path without the slash for demo purposes and prettier automatically reformatted some parts.

To test, first try to build current master with path change in a post. When running the build process, there will be an error as described in #31
Then, pull this branch, run yarn to get updated packages and try again.

[erquhart]

Good fix here, didn't realize we could work around the slash this way. I'd like to see this starter get to the point where the path field can be removed from the entry entirely and simply generated (that requirement is unique to Gatsby), but there are a few considerations there, probably better to attempt once decaporg/decap-cms#445 happens.