I was not removing context when I was leaving a tag. This caused the
context to grow indefinitely, which is not a valid representation of
where the placeholder was located in the template.
It could be useful to know the context when discovering a placeholder.
I also renamed MissingKeyEventArgs to KeyNotFoundEventArgs.
I created a new event KeyFoundEventArgs.
It could be useful to track which placeholders were found when parsing
the template. This event will allow keys to be mapped to different keys,
and support changing alignment and formatting.