Added option to override the report base path for resources

[svwu]

For our CI Integration, we need to specify the baseUrl where the Report loads resources from.
It may be an edge case but it's really necessary for us.

I opened an Issue aswell:
#1093

[svwu]

We fixed the problem in our CI with some workarounds - this PR is not necessary for us anymore.
If it is useful for others, it can be considered to be still merged or not.

[garris]

Thanks @svwu. Just curious how you solved the issue. Please post if you have a second. Cheers!

[svwu]

Hey garris!
It was a bit tricky, cause we use Docker Swarm and run different BackstopJS tasks for different environments including different backstop.json files.
We implemented a workflow which copies the files to a persistent place which wasn't accessible for our html report in Jenkins. Now we added a build step which also creates a symlink to the correct persistent folder. So we won't need a configureable static path which is accessible
.

[garris]

Got it. yes, this is very similar to what we are doing in our CI as well. Thanks for the update -- hope all is well! Cheers.