Improve color theme selection for navbars

[luyangkenneth]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [X] Enhancement to an existing feature

Fixes #386

What is the rationale for this request?
This PR allows users to customize their navbar colors. There is now an option to choose from 3 color schemes that are provided by each bootstrap/bootswatch theme.

There's also an option to specify type="none", for users who wish to inject their own CSS classes using our existing add-class feature.

In short, <navbar> now has type="primary" | "dark" | "light" | "none", instead of type="inverse" | "default".

What changes did you make? (Give an overview)

• Improve color theme selection for navbars vue-strap#102
• Updated all instances in docs and tests where we used a <navbar>
• Other minor cleanup

Testing instructions:

1. Look at https://deploy-preview-797--markbind-master.netlify.com/userguide/usingcomponents#navbars for the code and the corresponding output.
2. Change the navbar type to none or some gibberish value (this will default to primary), observe that the rendered navbar is styled correctly.

[luyangkenneth]

Using the default bootstrap theme

Using the bootswatch-united theme

[yamgent]

Propose a merge commit message. Also your associated vue-strap PR have been merged, use the latest master branch to replace vue-strap.min.js?

Btw a side note, actually PR authors shouldn't be the ones re-generating vue-strap.min.js, but right now we don't have a proper packaging system for vue-strap that is specialized for MarkBind, so I can understand the need of re-generating vue-strap.min.js by yourself. In the future, I foresee such updates to be merely just a change in the version number (i.e. modify package.json to up the version number of vue-strap).

[luyangkenneth]

Just did a force push 👍

Here's my proposed commit message:

Improve color theme selection for navbars (#797)

* Modify the Navbar.vue component to take in a `type` of
  `primary | dark | light | none` so as to allow users to select
  navbar colors based on each Bootstrap/Bootswatch theme.

* Update all <navbar> instances in docs and tests

[damithc]

Gave it a try. Nice 👍
Also discovered that add-class works even if the type is not none. e.g., type="primary" add-class="bg-danger" would give you a nice red navbar.