Feature: new provider positions

[besLisbeth]

Hi Reinaldo! I added the possibility to render alerts in additional positions:

• 'middle left',
• 'middle',
• 'middle right'.

I also added exported constants in options.js file which gives the opportunity not to type strings with positions, transitions, and types of alert. Moreover, a little bit restructure the test suites.

Waiting for the feedback:)

[schiehll]

Hey @besLisbeth! This is awesome, the codebase looks way better structured and future proof now, thank you!

Also, the tests are a lot better organised too, great work!

The only nitpick I have is the middle name...maybe it could be middle center to follow the pattern of top center/bottom center?