Add possibility set vendor basename

[vbryan]

Composer provide possibility to setup vendor installation path: https://getcomposer.org/doc/06-config.md#vendor-dir

Currently if vendor path != 'vendor' your library goes to infinite loop at:

while ( basename( $base_path ) != 'vendor' ) {
    $base_path = dirname( $base_path );
}

That PR allow setup vendor name on create new migrator instance.