Allow a template file to be set

[patcon]

It's not possible to generate all the config through channel names, so would be great to have a template that lives elsewhere, that wraps the auto-generated part.

[general]
RemoteNickFormat="{NICK} ({CHANNEL}@{BRIDGE}): "
TranslationAttribution=" [translated by :google:]"
IgnoreNicks="slackbot"

## ACCOUNTS

#[slack.example]
##Token to connect with the Slack API
##You'll have to use a test/api-token using a dedicated user and not a bot token.
##See https://github.com/42wim/matterbridge/wiki/Slack-bot-setup for more information
#Token="${SLACK_EXAMPLE_TOKEN}" # real user

[slack.foo]
Token="${SLACK_FOO_TOKEN}" # bot user
ShowJoinPart=true

[slack.bar]
Token="${SLACK_BAR_TOKEN}" # real user
# Too noisy. Related: https://github.com/42wim/matterbridge/issues/524
ShowJoinPart=false

## GATEWAYS & BRIDGES
{{AUTOGENERATED}}

[[gateway]]
name="some-other-gateway"
enable=true
  [[gateway.inout]]
  account="slack.foo"
  channel="general"
  [[gateway.inout]]
  account="slack.bar"
  channel="general"

Options:

• store on filesystem
• point to url (a gist or raw github repo file)
• get from envvar (likely base64 encoded)