Add Open-Mode to extends the possibilities of usages

[PaulRosset]

Description

The Open-Mode has the goal to extends the possibilities by using only the crucial information of the size of the build and nothing else, thus it permits to use this data to create a lot of other things and not only for a CI purpose in case of fail or success depending a maxsize.

Motivation and Context

Issue #253.

It extends the possibilities of usage of bundlesize by having only the crucial information of the build size.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] If my change requires a change to the documentation I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I created an issue for the Pull Request

Concerning the style of the project, a bug occur linked to prettier, I've created an Pull request that is related to #246

For the documentation, this feature is not certain to be accepted yet, so I will update the doc when we will be sure.

Thank's :)