Allow passing a logger to commands

[Reinmar]

That would allow avoiding sth like this:

.then( () => {
	return execCommand.execute( execOptions( `git checkout ${ repository.branch }` ) );
} )
.then( ( response ) => {
	log.concat( response.logs );
} )
.then( () => {
	return execCommand.execute( execOptions( `git pull origin ${ repository.branch }` ) );
} )
.then( ( response ) => {
	log.concat( response.logs );
} )

[Reinmar]

https://github.com/cksource/mgit2/pull/35#issuecomment-274782553 more here.