Ability to add filters, functions, and tests with dynamic names

[SergioBenitez]

At present, the Environment::{add_filter, add_function, add_test} methods make it impossible to add filter/function/tests with dynamic (String) names as they expect an &'source str. In my application, filters, functions, and tests are loaded dynamically, so I cannot possibly generate an &str for the names unless I go through some unsafety and keep self-references around. Alternatively, I can simply leak the String, but this is far from ideal.

Ideally, I could simply pass the String to the appropriate method and have that be it. It's not evident to me what's being saved here as a result of using an &str solely, so my proposal is to allow N: Into<Cow<'source, str>> instead, and store the name as a Cow<'source, str>.

[mitsuhiko]

I think this is very sensible. This also is an issue that needs addressing for #135 too.