Allowing plugins (or any external code) to override existing formats

[seeschloss]

I am writing a library that handles French revolutionary calendar dates.

When looking into how I could use moment together with it, I found that it's quite difficult for a plugin to define new formats. The jalaali plugin has to completely wrap moment in order to add its jYYYY and so on formats. In the process, it also has to reimplement (often just copying) internal functions, just because it can't access and modify moment's internal variables.

I tried a quick modification of moment's code to allow plugins to declare "extraFormats" that will then be handled just like the default internal ones, here it is: seeschloss@ff4920f

That's not a pull request because:

1. I'm not sure the moment project is interested at all in the idea
2. It's far from complete, doesn't provide any support for parsing these extra formats
3. Kind of a quick hack (especially the really ugly RegExp modification part) and could be done in other, probably better, ways

Basically, before doing anything better I'd like to know what you think of this idea? Is it worth it?

[ichernev]

Adding formats, tokens etc is part of adding pluggable calendar systems: #1454

PRs welcome.

[marwahaha]

Closing due to age. Still, a cool idea, please open a PR if you're interested.