Implement textexpander's macros

According to their website, textexpander proper has support for the following macros:

• %clipboard, which expands to whatever was in the clipboard
• %key:⟨keysym⟩%¹, which simulates pressing that key
• %|², the "cursor position macro", moves the cursor to that part of the expansion after the text is expanded.
• %\³, which selects from wherever %| was to "here".
• %%³, which expands a single percent character (escape)

... and a whole lot more. Honestly, I'd really like to support many more of these.