introduce more optional features to slim down required dependency graph

[HookedBehemoth]

This introduces the feature flags: logging, assets, post, session.

The goal was to slim down required dependencies in my application and the categories were chosen to make an impact on that. Updated tiny_http to get rid of a hard chrono requirement.

A simple hello world with default-features = false goes from 96 to 44 dependencies and total build time in release from 30s to 15s.