Allow setting different maxBuffer values for stdout/stderr

[ehmicky]

This starts implementing #930, starting with the maxBuffer option.

This is the beginning of a series of PRs to add the following syntax, to allow some options to have different values for stdout and stderr.

execa(..., {maxBuffer: {stdout: 10, stderr: 20}});

The other options are verbose (which is the most useful one in that respect, since users might want stderr to be more verbose than stdout during debugging), stripFinalNewline, buffer, lines and encoding. This PR adds the base for the next PRs, so it is bigger.