Allow to overwrite root and grid URLs

Add `--root-url` and `--grid-url` options to the cli.
This will allow to override URLs in config file, for example,
for CI purposes.
Document new CLI options
Add changelog entry about new options
Merge URL overrides options into master

New options allow to override `rootUrl` and `gridUrl` with
`--root-url` and `--grid-url` cli flags.