Switch to toml files rather than env variables about go HOT 3 OPEN

Initial attempt on top of docker-stellar-core-horizon's horizon.env:

[core]
url = "http://localhost:11626"
database_url = "postgres://stellar:__PGPASS__@localhost/core"

[config]
database_url = "postgres://stellar:__PGPASS__@localhost/horizon"
ingest = true
log_level = "info"
network_passphrase="__NETWORK__"
per_hour_rate_limit = 72000

@jedmccaleb is it ok to start from?

from go.

What's the reasoning behind this one? Env variables are more standard, and can easily be scripted.

from go.

In opinion we should do the same thing we do in Horizon: allow setting variables by toml file AND env variable. It's flexible (supports both users who need settings in one file and those that prefer env variables) and easy to implement (there's a package that allows this here: https://godoc.org/github.com/stellar/go/support/config).

from go.