Fix: Adds clamping to getSource beforeCount (fixes #1427)

- Without the clamping in place, there is the possibility of passing
  negative values to string.slice and winding up with odd results.
Docs: Fix broken code fences in key-spacing docs
Merge pull request #1429 from greggian/fix-getsource-clamping

Fix ESlint.getSource() clamping
Merge pull request #1440 from btmills/key-spacing-docs

Docs: Fix broken code fences in key-spacing docs
New: Add ability to time individual rules (fixes #1437)