Flag "for examples"

**The Error**
I was writing some text, and mindlessly typed "for examples", and was surprised that harper didn't flag it.

**Examples (2-3):**
1. "While it does uncover some bugs, it doesn't have the ability to generate stack traces for example."

**References**

- https://dictionary.cambridge.org/dictionary/english/for-example
- https://idioms.thefreedictionary.com/for+example

**Potential Edge Cases**

As [mentioned on Reddit](https://www.reddit.com/r/grammar/comments/3abpte/is_for_examples_ever_okay/), the sentence "She was looking for examples to back up her argument." is correct.

(This is the reason I'm opening a request instead of a pull-request, as I can't figure out how do handle this in a weir rule)

Start by locating Harper's existing rule for “for example” and read how its matching conditions are expressed, especially the referenced weir rule. Check the reported sentence and the “looking for examples” edge case, then add coverage showing that the standalone error is flagged while the valid phrase is preserved.

Written by the indexing model from the issue text.