Add case conversion filters.

[atholbro]

Hey,

Here's a quick little PR to add heck. This adds a few more text conversion options:

• upper_camel_case
• camel_case
• snake_case
• kebab_case
• shouty_snake_case
• title_case
• shouty_kebab_case

Maybe required?

• Perhaps a new example?
• Maybe a unit test?

[Keats]

It needs to be rebased.
Looking at the source of tera-text-filters, it's probably better for us to just depend on heck explicitely and add the few lines of glue without the extra dependency. We also don't need the docs for the filters in the source so thats literally maybe 15 LoC

[Keats]

Better to use tera_text_filters::register_all(&mut tera) and remove the import so it's clearer what happens

[atholbro]

Ya I like this idea.

And I might take a look at depending on heck like you suggest.

[atholbro]

Alright, it took me a while to get back to this but I've made the changes you've requested. The PR now depends on heck directly.

I put the filters in a new filters.rs file, happy to change the name or move them as I wasn't quite sure where you'd like these to live. I feel like there's probably a smarter way of defining these filter functions but I'm still new to rust so nothing really came to mind.

I also updated the README to include a bit about the additional filters.

[Keats]

Thanks!

[atholbro]

Hey, any thoughts on merging this one?

[Keats]

Woops sorry completely slipped my mind. I'll merge and do a release next week.