Allow ELResolver to be specified in Config

[mattcoley]

Alternative to #431

This will allow us to connect our custom ELResolver.

[mattcoley]

I wanted to keep the existing behavior consistent so kept this method in for the builder.

[boulter]

instead of making it part of the config, how about allowing it to be set on the Jinjava instance like it is for ResourceLocator. Then you can create whatever implementation you want.

[mattcoley]

The ELResolver used right now is determined by the config (through the readOnlyResolver setting). Is there any advantage to exposing the ELResolver at the Jinjava level? That would mean you would need a separate instance of Jinjava for every ELResolver you would like to use vs. being able to reuse the same Jinjava object with different ELResolver's specified by the config.

[mattcoley]

I realized I forgot to add the Builder method to the config for specifying your own ELResolver.

[boulter]

I think it should be possible to provide your own chain of resolvers. The readOnlyResolver seems a little too inflexible. Maybe you could just pass your CompositeELResolver to config if you don't want to add a setter to Jinjava.

[mattcoley]

The config accepts any ELResolver so you can pass in your own CompositeELResolver to the config. The readOnlyResolver boolean was an existing config value that I didn't want to remove for backwards compatibility, I just moved the logic to map the boolean value to one of the two default values to the config from the JinjavaInterpreterResolver