Add Officer Positions to Member Pages

[JoeDailey]

Information like this can be added to the frontmatter of semester-pages. For example:

# _members/2019/spring.md
---
semester_index: 3
president: Matthew Kramar
# Other roles are placed here
members:
  # All other members
---

Then members_layout.html can be update to display this new information.

[JoeDailey]

@ajkofman, let me know if you have any questions!