Update baseURL on forks

[lukaszstolarczuk]

Our web sites, built from forks do not work 100% properly - links ref to "pmem.io" instead of "myfork.github.io", which is not ideal for testing and providing a preview (for review, on PR). It's because of hardcoded baseURL in our config.toml and CNAME (?).

Ref. to the discussion in PR: #181 (comment) and mentioned there a link with a proposed solution:
rstudio/blogdown#412