Make the dependency on counsel-imenu optional

[akirak]

This aims to resolve #610. It allows the user to replace counsel-imenu with something else by customizing lispy-imenu-function variable.

Note that the user will have to install counsel separately to use the default setting. I am not sure if this is right. Please feel free to amend this PR.

[aaronjensen]

This is great, if this gets accepted it would be nice to do a similar thing with swiper.

[akirak]

This is great, if this gets accepted it would be nice to do a similar thing with swiper.

@aaronjensen Thanks for the comment, but I don't think it is possible with swiper, due to its tight integration with ivy.

I am not still sure if @abo-abo will accept this PR. Because the dependency on counsel is removed here, it will become necessary for users to install counsel separately. It would be nice if there were a better way to declare the dependency.

[aaronjensen]

@akirak Right -- ideally ivy would be optional too, but I realize that's likely a bridge too far.

[benjamin-asdf]

I second making ivy optional as well. Isnt't the only usage lispy-occur?

[akirak]

I now don't mind having the ivy dependency. If you don't like this change, please close the PR.

[abo-abo]

Merged, thanks.