Allow multiple exes, baselines

[mattip]

PyPy ~needs~ wants to show historical comparisons on its home page of

• cpython benchmarks, with the first one the reference for the others. These are called baselines
• pypy benchmarks: all tagged revisions and the latest build ("nightly"), these are called executables

So we allow multiple baselines and executables by putting this in the settings:

DEF_BASELINES = [
                 {'executable': 'cpython', 'revision': '3.7.19'},
                 {'executable': 'cpython', 'revision': '3.11.9'},
                ]
DEF_EXECUTABLES = [
                   {'name': 'pypy3.10-jit-64', 'project': 'PyPy3.10'},
                   {'name': 'pypy3.9-jit-64', 'project': 'PyPy3.9'},
                  ]

I tested this locally.