Added defer_update flag to add_stock function

[nuvious]

Added defer_update flag to add_stock function so bulk adding of stocks can have update deferred until after all are added.

This resolves issue #57.

Unit tested using python:3.10-bullseye docker image.

[fmilthaler]

CI pipeline was successful (as expected) :)

Thank you very much for your interest in the project and contribution, it's much appreciated. And thanks for the 3+ years patience of this change to get into the project. ;)

Two minor changes:

1. You can add your username (and name, if you are comfortable with that) with a short description of your contribution to the file CONTRIBUTORS.md.

2. Please change the target branch to feature/defer_update. Reason for that is, that I recently added scripts that automatically run in GitHub Actions and update the version and README files accordingly (with new release number). It would have done the same in this PR, but it determines new version numbers based on the source branch name of the PR. Example:

   • source branch name starts with feature/: version number is incremented by 0.1
   • source branch name starts with bugfix/: version number is incremented by 0.0.1

   Since this PR merges from source branch name master (into master), it didn't make those changes. By merging into feature/defer_update now, we can simply put up another PR that merges from feature/defer_update into master, and those changes are automatically taken care of.

[fmilthaler]

@nuvious I'll merge this into feature/defer_update within FinQuant now and will make the last minor modifications on that end. Following that, I'll merge it into master and your contribution will finally see the end of the tunnel ;) Just let me know if you want me to list you by your GitHub username only, or username + Name in the file CONTRIBUTORS.md