Add a set of click based cli utilities to simplify writing launch scripts

[sphuber]

Fixes #1193

Note: I know Rico has been working on porting verdi to click at which point part of this may be tackled, but until that time I really need this functionality in different plugins that is currently included in aiida-quantumespresso.

These utilities simplify the writing of command line interface scripts
that are very useful for plugin developers to write launch scripts with
minimal effort. It provides a decorator @command that will turn a
function into a click command that automatically loads the database
environment. It also provides various other @option like decorators
which predefine commonly used input arguments and their validators for
AiiDA types such as Code, StructureData and Calculation. For
example a minimal script that will take a structure data id and print
its cell would look like:

# -*- coding: utf-8 -*-
import click
from aiida.utils.cli import command
from aiida.utils.cli import options

@options.structure()
def launch(structure):
	click.echo(structure.cell)

[giovannipizzi]

Awesome! Could you add some documentation for developers (form the PR message) and convert at least one command to this format, so we have an example of how it works?

[sphuber]

I explicitly did not add documentation here, because I wasn't sure whether this would stay in in this form for a long time, given that Rico would be porting the verdi stuff and he might do it differently. Adding docs now, sort of guarantees that it is going to be supported like this for a while. But if you think that is fine, than I will gladly add some documentation.