Add CLI entrypoints for module and program invocation

[nicholasjng]

Supports nnbench <benchmark> <options> as well as python -m nnbench <benchmark> <options>.

Like pytest, we give the option to omit the target location, in which case it becomes a directory named "benchmarks".

Supports a very rudimentary context building from switches method, and tag filtering via switch.

[nicholasjng]

Test setup:

# example.py

import nnbench

# no idea yet how to hand off args on the command line, so we need them hardcoded.
@nnbench.benchmark
def add(a: int = 1, b: int = 2) -> float:
    return a + b

Then, nnbench example.py or python -m nnbench example.py does the trick. Bonus: If you put the file into a directory named "benchmarks", even nnbench without any args works.