Consider making --always-install the default

I was poking around the code and discovered this great --always-install. It doesn't show up on the top level of help (and I previously didn't realize it shows up with no description if you do `solc-select use --help`.

But personally I think solc-select should --always-install by default and we change this option to --never-install

Either way, I think it would be nice to have this option listed on the top help screen.