Expose all property

[jaylinski]

Description

Exposes the all property. (Although I would prefer calling it listeners.)

This makes it possible to clear all events by executing mittInstance.all.clear().

Is a (lightweight) alternative to #104.

Relates to

#70 #72 #102

[jaylinski]

Error: compressed-size-action was unable to comment on your PR.
This can happen for PR's originating from a fork without write permissions.
You can copy the size table directly into a comment using the markdown below:

Size Change: +11 B (1%)

Total Size: 941 B

Filename	Size	Change
dist/mitt.es.js	216 B	+3 B (1%)
dist/mitt.js	215 B	+2 B (0%)
dist/mitt.modern.js	215 B	+3 B (1%)
dist/mitt.umd.js	295 B	+3 B (1%)