Add execa.command()

[ehmicky]

This PR is a follow-up on #182 which allowed specifying the command and its arguments as a single string.

It improves the common case where the program contains a space in its path. For example on Windows C:\Program Files\node\node.exe.

With execa(command, [array], [options]), when:

1. command contains a space
2. array is defined
3. options.shell is not true

Then:

• former behavior: an exception is thrown.
• new behavior: command is handled as a command with a space in its path.

// Success
execa('echo hello')
// Fail
execa('echo hello', ['world'])
// Success
execa('echo hello', {shell: true})

// Fail
execa('C:\\Program Files\\node\\node.exe')
// Success
execa('C:\\Program Files\\node\\node.exe', [])
// Success
execa('C:\\Program\\ Files\\node\\node.exe')

I've fixed the tests. No changes need to be done to the README since it already implies that no spaces escaping is needed for programs containing a space in their path.

[GMartigny]

I think this should be documented somewhere that [] is required as second argument when your command has spaces in it.

Weird behavior I see while testing:
If I want to escape myself the command execa("fixtures/command\\ with\\ space") it fail anyway. Maybe it's not an expected behavior, just saying.

[ehmicky]

Good point! Fixed.

[GMartigny]

I confirm that there's a bug with path with more than one spaces:

execa("path/with/one\\ space"); // Success
execa("path/with/more \\than\\ one\\ space"); // Fail

This is due to previousToken not being accurate when ignoring some item of the array.

[ehmicky]

This is a different bug. I've opened a PR at #262 to fix it.

[ehmicky]

Done. I've separated the command feature into two new methods execa.command() and execa.commandSync().

[ehmicky]

Fixed!

[ehmicky]

Fixed.