Update stack lts version on fly

[willbasky]

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.

[chshersh]

@willbasky In the beginning, Summoner used stack new command to create Stack-based projects, and this command fetches the latest LTS. However, this took noticeable amount of time during project scaffolding so we removed this option.

But the more severe concern is that new LTS can come out using newer major GHC before we test it in Summoner and update the code with relevant changes. And this will definitely break all projects created with Summoner after that. In other words, even with an option, you need to implement something smart to use "the correct latest LTS".

[willbasky]

Thank you for constraints. LTS should be bounded to last tested GHC and not takes much time. I wonder does stackage have API? stack likely interact over it, doesn't it?