Support for post-process complex algorithm

[vincentsarago]

Following #487,

I think titiler is missing a key feature which is the support for complex post-process operation (e.g hillshading). Right now we can use expression to apply simple arithmetic operation but it's really limited to what numexpr can handle (https://numexpr.readthedocs.io/projects/NumExpr3/en/latest/user_guide.html#supported-operators).

In rio-viz (a titiler local like plugin) I've started a PR that sketches what it could be: developmentseed/rio-viz#51

[vincentsarago]

on going work over #542