Add new `--build` flag for CLI

[jwoertink]

Add a --build flag that will build that crystal script in to an executable like lucky --build gen.migration => bin/lucky.gen.migration.

Related: #869

[jwoertink]

The one thing I have a question about is, should this flag be called before executing the script, or do we have it do a build and run at the same time?

The difference here would be if the arguments to gen.migration (for example) would be required or not. If this just does the build, then they wouldn't be used; however, if it's a build and run then they would be required..

[jwoertink]

I think instead of a --build flag, it should just be an arg so it can take flags itself.

lucky build gen.migration
lucky build --release gen.migration