Allow to set custom tags as "hidden"

[pkamps]

Background
You want to be able to hide ezoe custom tags in the UI - so that editor cannot use that custom tag anymore. But you don't want to completely remove the custom tag because your content already is using that custom tag. The solution is to allow to hide specific custom tags. Something like making them "deprecated".

Solution
In the settings you can specify which custom tag is hidden and it won't show in the editor UI anymore.

Testing instructions

• Change the settings and mark a custom tag as "hidden"
• Go to the admin UI and confirm that this custom tag is not available anymore in the dropdown box

[peterkeung]

We needed this on AMNH as well, and implemented a solution fully in templates. This is cleaner, so +1 in general. However, the INI documentation should be improved so that it's clear that you put the identifier as the key and "true" (or "false") as the value. Then this so go to upstream as well.

[pkamps]

Thanks for pointing that out. I don't like how you had to specify the setting (very uncommon). So I decided to simplify the settings. I also added an example in the settings file.

[pkamps]

Ready to get merged.