feat(BuildCommand.php): add lifecycle hooks for prehtml and prepdf

[bmcminn]

Adds user editable functions to ibis.php that allows users to modify
content via global event hook callbacks.

Fixes issue #13

[themsaid]

I think it's better if you don't have to define global functions. Just include a closure in the config file to be invoked within the build command. Also can you please include an example of how you're using both callbacks in a real scenario?

[bmcminn]

Yeah it's not ideal.

Reason I went this route was because the method signature for BuildCommand.php@buildHtml() doesn't have an argument for $config.

I think it makes more sense to make $config a property of the BuildCommand class and initialize it in the execute() method so we can just reference that instead of passing the $config value around from method to method. I'll get that worked in my next push.

As for an example:

function prehtml($markdown) {

    // remove frontmatter/comments/notes/etc
    $markdown = preg_replace("/-----/", '<hr>', $markdown); // convert horizontal rules first
    $markdown = preg_replace("/---[\s\S]*?---/", '', $markdown); // strip frontmatter content blocks
    $markdown = preg_replace("/\/\/[\s\S]+?\n/", '', $markdown); // strip inline C-style comments

    return $markdown;
}

The prepdf() method could be used in the same way for preprocessing the resulting HTML, like custom flags similar to the page [break] marker. This allows users to extend the render process as they see fit without the core library having to support all their edge case needs. If someone wanted to use a MathML library to convert their equation syntax then this gives them an opportunity to solve that themselves rather than needing Ibis to support it for them.

It also gives people a place to initialize content plugins at will, so my comment stripper could become a ibis-comment-stripper macro plugin that I initialize in my ibis.php file for portability. It's really up to the person what all they would want to do in there, but the core library doesn't have to do anything else to directly support that.

[bmcminn]

I think that's the final version of this feature.

To recap, I moved the $config variable from being a local method argument to being a private class field of BuildCommand class. I also updated each local $config reference to $this->_config and removed all $config arguments from the method signatures to avoid ambiguity.

So far I'm pretty happy with how this turned out.

[bmcminn]

I think this feature is in pretty good shape for final review.

[bmcminn]

@themsaid any thoughts on accepting this PR?

[bmcminn]

@themsaid *bump