[feature request] support mime-view-mode

[showgood]

When view email in WanderLust, its major mode is mime-view-mode.

As often there is some links in the email, it would be really helpful if ace-link can support mime-view-mode. thanks a lot!

[abo-abo]

Please check if an existing function already works in mime-view-mode. For example, ace-link-gnus.
We can then add it to the ace-link command so that mime-view-mode is automatically detected.

[showgood]

I have tried ace-link-gnus, unfortunately it doesn't work for mime-view-mode.