Default mapping for dictionary lookup

[wikimatze]

The current one if «CTRL-X» «CTRL-K». In my eyes `«CTRL-X» «CTRL-D» (for Dictionary) would be more suitable. What do you think?

Bests from Berlin.

[reedes]

Ctrl-X Ctrl-K is the vim default, so I'm reluctant to override that.

However, we could provide the mnemonic-friendly mapping example in the README, if you'd like to submit a patch.