Customizable subplot title font

[Zomtir]

Description

It is a very minor feature. It makes the default font size (16) customizable. Using relayout!(plt, font=attr(size=18)) does not affect the annotations of subplots, therefor this parameter would be valuable to adapt the font size accordingly.

This should also fix #985 properly.

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the code generator and not the generated files.
• I have added tests or modified existing tests.
• For a new feature, I have added documentation examples (please see the doc checklist as well).
• I have added a CHANGELOG entry if changing anything substantial.
• For a new feature or a change in behavior, I have updated the relevant docstrings in the code.