Changed workings of PUT and USR to build better 5.25" POP images.  Added BLT and BGE.

* Extended the PUT directive to allow a certain number of lines to be skipped at the beginning.  And, PUTs can now be nested. This allows me, for example, to do this to skip the first two lines of `GAMEBG.S` which set `ThreeFive=1` by doing this:

  ```
  ThreeFive = 0
     put GAMEBG,,,2
  ```

* The USR directive now it takes the output filename stem from the _top-level_ source file, rather than the source file containing the USR.
* Added BLT and BGE mnemonics.
* Updated the snap documentation accordingly.