Add documentation about custom filters

[djc]

[djc]

I should also document the list of built-in filters.

[Cokemonkey11]

Would be great to have a more complete example of a custom filter.

[djc]

Fair point! For now, have a look at this test case:

https://github.com/djc/askama/blob/master/testing/tests/filters.rs#L39

[Cokemonkey11]

Thanks! I managed to make some filters with this test case as reference:

/// Any filters defined in `mod filters` are accessible in your template documents.
mod filters {
    pub fn extension(s: &str) -> ::askama::Result<String> {
        Ok(s.split('.').last().expect("Couldn't split by '.'").into())
    }

    pub fn uscore_to_space(s: &str) -> ::askama::Result<String> {
        Ok(s.replace("_", " ").into())
    }
}