Allow template strings with all methods

[ehmicky]

Fixes #891.

Template strings with all methods

The template string syntax can now be used with all methods.

import {execa} from 'execa';

await execa`echo foobar`;

Non-template strings with $

Also, $ can now use the non-template string syntax.

import {$} from 'execa';

await $('echo', ['foobar']);

Default options

Therefore, the only difference between execa, $ and execaNode are now their default options:

• execa: normal default options, general use case.
• $: {stdin: 'inherit', preferLocal: true} default options, intended for scripts.
• execaNode: {node: true} default options, intended to run Node files.

execaCommand()

execaCommand() is slightly different from the other methods, and is only intended for cases like a REPL. In other cases, the other methods are better.

import {$, execa, execaCommand} from 'execa'; 

const runRepl = async command => {
  await execa(command);
  await $`${command}`;
  await execa`${command}`; // Those three commands do not work. This is parsed as 'echo foo bar' file, with no arguments
  await execaCommand(command); // This works.
};

await runRepl('echo foo bar');

Options binding

The template string syntax uses the following convention to set options:

await $(options)`command`;

Therefore, it requires adding $(options) binding. So this PR also enables options binding with the same syntax as $. This is quite nice as options binding with Execa was somewhat tricky. For example, to set global options:

import {execa as execa_} from 'execa';

const execa = execa_(options);

Which allows doing:

await execa(options)`command`;