Expose Web Client Options

[iancward]

Description

Describe your issue here.

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
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• I've searched for any related issues and avoided creating a duplicate issue.

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Enhancement

It looks like, while the RTM client is configurable with HUBOT_SLACK_RTM_CLIENT_OPTS, this module does not expose any way to configure the web client.
The code merely hard codes maxRequestConcurrency to 1 and provides no opportunity to enable other options, such as debug logging.

See https://github.com/slackapi/hubot-slack/blob/master/src/client.coffee#L29-L30.

Can something like a HUBOT_SLACK_WEB_CLIENT_OPTS environment variable be created and used when the web client is being initialized?

[iancward]

Also, is there any reason why the concurrency is set to 1? The documentation seems to indicate the default is 10 https://slack.dev/node-slack-sdk/web-api#request-concurrency.

[iancward]

Based on the underlying node-slack-sdk using a default of 10 for max request concurrency, it might be best to remove the setting entirely from this code and fall back to the whatever the node-slack-sdk uses for default.