Changelog headings and their content

[consideRatio]

When using the CLI to generate a changelog for chartpress, I found:

1. Closed PRs includes both merged and closed PRs. I think that a changelog should distinguish these or if keeping it slimmed omitting the closed ones. I ended up needing to check what PRs had been merged and what had been closed.
2. Issues that had been opened and closed were listed under both opened and closed. I think it would make sense to only list them once under closed if they are both opened and closed.

Here is a structure suggestion, what do you think @choldgraf?

Currently

• Other closed PRs
• Closed issues
• Opened PRs
• Opened issues

Suggested

• PRs
  • Merged
  • Closed
  • Opened (excluding closed/merged)
• Issues
  • Closed
  • Opened (excluding closed)