Add missing patterns and reordered gitignore.

- .lein-repl-history exists after running lein repl
- .nrepl-port exists while running lein repl
- .lein-failures exists after running lein test
- order is taken from: https://www.gitignore.io/api/clojure
Use existing function for adding icon to menu link.
Refactor nav-links function.

Improving readability by extracting some methods and function local
variables.
Add client side opt in for image inlining.

Refers to #92.

Currently to preserve behaviour image inling is turned on by default and
the user has no option to disable it. This will be added later.
Add preference section to navigation box.

Fixes #92.

This enables the user to opt in for image inlining. By default image
inlining is disabled.
Merge pull request #92 from mvitz/inline-image-option

Add client side opt in for image inlining