Added dictionary definitions

[dawsers]

This PR can add dictionary definitions in the documentation window for each candidate item when selected. It is off by default, and the README.md file shows two examples of how to enable it with different dictionary definition sources.

[f3fora]

Even if I wanted to keep this plugin as simple as possible, I like your idea, However, I need you to do some changes before merging, and use a consistent format style.

[dawsers]

OK, let me know what you want me to do if you want to merge the PR. But I understand if you don't, no worries.