Update features.rst and HISTORY.rst

[whzup]

Description

Added the GeneralOptimizer and the plotters module to the features.rst
file and provided a deprecation warning in the environment package.
Updated the HISTORY.rst file with the new release note format based on
the notes in #180.

Related Issue

Motivation and Context

Makes the HISTORY.rst file look prettier.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

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

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests pass.

[ljvmiranda921]

LGTM! Thanks for this @whzup ! I will merge to master today so that tomorrow I just need to create the wheels and tarballs and upload to PyPI.