Triage (CLI) configuration and pluggability

*Note*: I realize that this issue should be divided in several subissues. Nevertheless I am creating it because I wanted to have the original @jesteria post in one place accessible to all)

(All the following is from the @jesteria's slack post):

***My project***

```
/
└── experiment.py
```

**CLI**

`$ triage [-s|--setup PATH] ...`

* CLI looks for and optionally loads Python module experiment in CWD
i.e. argument --setup defaults to `os.path.join(os.curdir, 'experiment.py')` -- ...IFF this file exists
* If setup desired, but at different path, can specify `--setup`, or envvar `TRIAGE_SETUP`

**Setup module**

Probably should just be a general entrypoint, allowed to execute arbitrary code, (rather than expected to merely define settings, like Django's settings.py).

* User gets (optionally) a Pythonic entrypoint, e.g. for setup of SQLAlchemy listeners (yay!)
* Triage gets (optionally) functional specification of its configuration

E.g., `experiment.py:`

```{python}
from sqlalchemy.event import listens_for
from sqlalchemy.pool import Pool

from triage import config

@listens_for(Pool, "connect")
def my_on_connect(dbapi_con, connection_record):
print("New DBAPI connection:", dbapi_con)

config.update(
# no way I'm going to set this here -- triage.config respects envvars!
# database_url=...,
...
)

# I want an easy, bulk way of updating keys, without overwriting entire dictionaries (as __setitem__ and update would do)
# (Note, we could probably achieve the same with `update`, so this interface really isn't necessary, depending on how people like it.)
config.set(
temporal_config__feature_start_time='1905-01-01',
...
)

if os.getenv('PUDDING') == 'plentiful':
config['eat'] = 'it-all'
```

**Triage config**

* provides dict interface
* but backed by reasonable defaults: user setttings > envvar settings > library defaults
* Perhaps CLI imports triage.config, after loading experiment.py, and uses config to instantiate lower- level triage classes

We get:

* Pythonic pluggability in CLI
* the centralization of configuration
* a place to insert envvars
* lower-level classes unaffected by these concerns (of envvars and configuration, etc.)

Start with the CLI entry point `triage` and the referenced `experiment.py` setup module, then inspect how `triage.config` is currently defined and used. The issue is an umbrella for configuration, environment-variable precedence, setup-module loading, and pluggability; it is done only after these concerns are split into concrete, agreed subissues.

Written by the indexing model from the issue text.