Only update api if specifes the name

[mgi166]

The reason why I create this PR

Recently, updates aki017/slack-api-docs@1ac07b4 by aki017/slack-api-docs-generator.

But aki017/slack-ruby-gem does not follow the latest aki017/slack-api-docs. So if you execute rake api:update, creates too many diffs in one commit.

It's undesirable to revert api update if you failed.

I think that if rake api:update specifies api name, only update the api, so prevent the tragedy and you can more easily update aki017/slack-ruby-gem in stages

I hope that aki017/slack-ruby-gem follows latest slack api as soon as possible.

Usage

# No specifies the argument, update all.
$ rake api:update

# Specifies the api name, only updates the api.
$ rake 'api:update[channels]'

What do I ask

What do you think this policy? Please let me oponion.

[miguelperez]

This seems reasonable. @aki017 are you planning on updating the gem with the latest Slack API calls?