Add additional parameter to sepia effect to control its intensity

[orhan]

What's Changing and Why

This will add an additional parameter to the sepia effect to control its intensity.
Useful for implementing some Instagram-like filters via Jimp.

What else might be affected

Nothing, still works as expected if you call, as the f parameter is optional.

Tasks

• Add tests
• Update Documentation
• Update jimp.d.ts
• [] Add SemVer Label