Support HTML within toast message

[patrick-lewandowski]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/mobi/goponents/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe: Chore

What is the current behavior?

HTML passed into message of toast would render as string instead of HTML.

Issue Number: #239

What is the new behavior?

HTML passed into message of toast will render as HTML. Executable HTML is not supported!

Does this PR introduce a breaking change?

• Yes
• No

Other information

Closes: #239

[grahamhency]

@patryk0605 nice work, thanks for adding the docs too!