Update stack lts version on fly

When I set project with summoner I have to update stack's lts version to last one manually. (open stackage.org and edit stack.yaml)
I suggest to make summoner to fetch last lts version on fly as option. Of course who need particular version it rest as an option.

Start by tracing how summoner generates and edits stack.yaml, then inspect how project options are represented. Check the stackage.org interaction and existing configuration flow; done means an option can select the latest LTS while preserving an explicitly requested version.

Written by the indexing model from the issue text.