Describe the bug
Col function is missing the ability to change the order of cols based on class.

order-*
order-sm-*
order-md-*

https://getbootstrap.com/docs/4.0/layout/grid/#order-classes

Use low-cost formatter such as variable expansion, sprintf and vsprintf instead of single-quote concatenation.
https://stackoverflow.com/questions/7147305/performance-of-variable-expansion-vs-sprintf-in-php

Curious, would it not be better when an element has a large number of options e.g.

custom_link($element, $links_to, $content, $style, $disabled)

as an array like: -

$args = array('I contain' => 'all the options');
echo custom_link($links_to, $args);

That way when working with lots of custom_links you have one central place to control them?

Describe the bug
Within the init_meta function you set a static value for the robots meta tag. Found here:

From an SEO perspective, it's less than ideal that this package sets all pages to index,follow via a robots meta tag. This is a meta tag that should be used with caution and is generally adjusted on a page/URL by page/URL basis.

For example, some pages may necessitate: index, nofollow, noindex, follow or noindex, nofollow. Any premutation of these could exist

To Reproduce
See:

Expected behavior
A user should be able to control the values this tag provides.

Describe the bug
It appears that the main bootstrap.min.css file is being provided by using a URL that points to your domain. This will provide a poor user experience to developers and website end-users.

The reason being that this isn't an established CDN, so this raises two huge red flags. The first being that the chance of this providing a cache hit across multiple Bootstrap using sites is slim to none. Meaning the main benefit of using a CDN - higher chance of asset cache hits - is moot. And the other main benefit - files being delivered via regionally located servers - doesn't apply.

TL;DR: A domain behind CloudFlare !== a real asset CDN.

Suggested Fix
Utilize the official BootstrapCDN or CloudFlares CDNjs.