Plugin config through chat cannot contain a line break after the plugin name

[sheluchin]

I am...

• [x] Reporting a bug
• [ ] Suggesting a new feature
• [ ] Requesting help with running my bot
• [ ] Requesting help writing plugins
• [ ] Here about something else

I am running...

• Errbot version: 5.1.1
• OS version: Ubuntu 18.04.1
• Python version: 3.6.6
• Using a virtual environment: yes/no

Issue description

When running !plugin config <plugin-name>, you get a help message:

Default configuration for this plugin (you can copy and paste this directly as a command):

When configuring a plugin through chat (Slack backend), there can't be a line break after !plugin config <plugin-name>. This means that, because it contains a line break, you cannot copy and paste the default configuration template as the message suggests. If you put the configuration dict on the same line, all works fine; if you put the config on new line, you get:

Unknown plugin or the plugin could not load <plugin-name>.

Steps to reproduce

Configure a plugin from chat using the Slack backend (I'm not sure if applies to all backends), making sure to include a line break between the plugin name and config dict, like:

!plugin config foobar
{...}

You will get an error: Unknown plugin or the plugin could not load <plugin-name>.

Now remove the line break and try configuring again:

!plugin config foobar {...}

The configuration will succeed.