Standards checklist:

• The PR title is descriptive.
• The PR doesn't replicate another PR which is already open.
• I have read the contribution guide and followed all the instructions.
• The code follows the code style guide detailed in the wiki.
• The code is mine or it's from somewhere with an MIT-compatible license.
• The code is efficient, to the best of my ability, and does not waste computer resources.
• The code is stable and I have tested it myself, to the best of my abilities.
• If the code introduces new aliases, I provide a valid use case for all plugin users down below.

Changes:

• Support disabling conda prompt display via CONDA_ENV_DISABLE_PROMPT
• Support customizing the conda prompt prefix via CONDA_PROMPT_PREFIX
• Change foreground and background colors to constants, and support defining them via AGNOSTER_CONDA_BG and AGNOSTER_CONDA_FG
• Add a comment: Command to disable the native conda prompt.