rename `exclude` to `ignore` and alias `exclude` to `ignore`

[boneskull]

as per #2036, "ignore" functionality is available via the exclude option of a .mocharc.* config file. Since ignore is more of an obvious (and precedented) option name, let's prefer ignore as the canonical name and alias exclude to it.

[boneskull]

(excludes is not actually an alias of exclude, which is fine.)