Improved templating

Conda smithy has a hardcoded number of files in https://github.com/conda-forge/conda-smithy/blob/master/conda_smithy/configure_feedstock.py which are generated into the resulting feedstock. Remove the bespoke functions, and just generate all files in the https://github.com/conda-forge/conda-smithy/tree/master/templates directory.

For feedstocks which need to override the templates, allow a templates directory in the feedstock which can inherit from conda-smithy (e.g. `{% extends "conda-smithy/README.tmpl" %}`) for truly bespoke customisation.

_Requires some knowledge of Jinja2._