Make Indigo a tool

Clarification and motivation

Indigo is currently only a library, meaning it can be used only as part of another project or using ghci (which is why the tutorial is based on this).

However, it would be nice to have Indigo as an executable tool as well.

Such application needs to be standalone and, for starters, be able to replace the stack ghci :indigo-repl from the tutorial in all its (non-interactive) uses, namely: compile an Indigo contract to Michelson and print it (to file as well).

The tutorial should also be updated accordingly, not removing the interactive option, but adding the (preferred) tool one.

Acceptance criteria

Beside being a library Indigo is a (minimal) usable tool.

The tutorial explains how to make use of it alongside the interactive :indigo-repl option.