Makes Attachments Optional, Other Config Options

[alanedwardes]

This ended up being more of a refactor than I planned, but I made the following changes:

• Exposed ShowDefaultAttachments, ShowExceptionAttachments and AttachmentColors in a new SlackSinkOptions object
• Renamed SeqLoggerConfigurationExtensions to SlackLoggerConfigurationExtensions
• Removed dynamic from JSON serialisation
• Overrides Dispose() on the batch sink, so HttpClient can be disposed

The attachments toggling was the main thing I wanted to add, the messages were too big on Slack - I could fit 5 log messages on my screen at once.

[mgibas]

Will take a look at this tomorrow, though quick question: is it compatible with previous version ? Thanks 👍

[alanedwardes]

Thanks. It is yeah, existing overload is still there, it calls the new extension method which accepts the options object.

[mgibas]

All looks good for me - please just update README.md with some example.

[phnx47]

I merged it and added example.