Support custom templates

[ankushg]

Is your feature request related to a problem? Please describe.
I love hajimari because it's so lightweight and easy to add ingresses to the dashboard

At the same time, some parts of the dashboard don't work great for me

e.g., I don't use bookmarks. I'd like my applications to be grouped rather than bundled. It'd be nice to edit the CSS or themes.

Describe the solution you'd like
It would be silly to expect @toboshii to fix all of these things, and it's a lot of work to build a ton of options into hajimari while maintaining its lightweight nature.

Instead, it would be super cool if there was a way to just… override the index.html.tmpl file or anything else under web/.

Ideally, there would also be a way to specify a map of additional properties for Apps via annotations and values so we could reference them from custom templates.

Maybe a custom: map[string]interface{} added to the App, CustomApp, and/or Link structs?