Add support for extracting history, procedures and plain notebooks

readpxp got a new logical parameter "ExtractText" which controls the extraction.

It creates the following character vectors:
- $mainproc   (main procedure)
- $history    (experiment history)
- $recmacro   (recreation macro)

And the following for all existing embedded procedures and plain notebooks:
- $Proc0      (embedded procedure named "Proc0")
- $Notebook0  (embedded procedure named "Notebook0")

Formatted notebooks are currently ignored.

No end-of-line conversion or similiar is performed on the text.

Signed-off-by: Thomas Braun <thomas.braun@virtuell-zuhause.de>

PS: I'm fairly new to R so any suggestions for further enhancements are welcome.