Closes #260 by filtering html from post excerpts

- The issue was that post excerpts would look inconsistent depending on
  whether they used the <!-- more --> tag or a manual excerpt
- This commit filters whatever html may or may not be in an excerpt and
  wraps the excerpt in a <p> tag. The consequence is that all excerpts,
  even if they span multiple lines will be displayed on the homepage or an
  archive in a single `<p>`
- closes #260 and #259