Add support for in-memory diagram generation using BytesIO

[rithvikgrandhi]

This PR introduces the ability to generate diagrams in-memory using a BytesIO object. This is useful for applications that require diagram generation without writing to the filesystem, such as web services or cloud functions.

Changes

• Added an output_buffer parameter to the Diagram class.
• Modified the render method to write to the output_buffer if provided.
• Updated the __exit__ method to handle in-memory diagrams.

Testing

• Verified that diagrams can be generated and rendered in-memory.
• Ensured that existing functionality remains unaffected when output_buffer is not used.

Example Usage

from diagrams import Diagram
from io import BytesIO

output_buffer = BytesIO()
with Diagram("Example", output_buffer=output_buffer):
    # Add nodes and edges here
output_buffer.seek(0)
# The diagram is now in the output_buffer