[#269] Make Indigo a tool

Description

Problem: 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).

Solution: Make indigo executable that could create a boilerplate, wrap around stack for build, run, and repl commands, and wrap around lorentz ContractRegistry commands.

For the docs part, this will be handled in other issues:

• #270 (closed) will explain the usages of indigo repl instead of stack ghci :indigo-repl in the tutorial.
• #232 (closed) will explain the usages of all commands of indigo to develop a contract.

Related issue(s)

Resolves #269 (closed)

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