Use python emoji package to support :emoji: format emojis

[jongracecox]

Is your feature request related to a problem? Please describe.
Make it easier to include emojis in badges by allowing :emoji: text in labels and values to be converted into the emoji character.

Some developer IDEs and consoles make it difficult to include emojis, so this would make it easier.

Describe the solution you'd like
Add new dependency: https://pypi.org/project/emoji/
Use emoji.emojize() on labels and values to switch emoji text to an emoji character.
Ensure that emojize is run before calculating badge dimensions.

Describe alternatives you've considered

• Embed emoji svg characters directly into the project
• Allow images / other svgs to be embedded

Additional context

Relates to #43 and suggested by @roipoussiere