Option iconClassMap does not work

[tuxpoldo]

Currently the iconClassMap option has absolutely no effects, because the values of the built-in iconClassMap variable are directly inlined in the toolbarBuiltInButtons during script initialization which occurs before the constructor call where options are passed.

To Reproduce
Steps to reproduce the behavior:

<textarea></textarea>
<script>
const easyMDE = new EasyMDE({iconClassMap: { bold: 'fa fa-italic' }});
</script>

Expected behavior
View the italic icon on the button for bold

Additional context
The mapping and the option were introduced with #454. Unfortunately the author defined the option but forgot to add the implementation.
The user @MrMicky-FR noticed and fixed the bug with PR #501 but no-one cared about merging it.