allow command line options in data files

[simonmichael]

Ledger allows command-line options to be written in the journal file, like directives. This might be nice to have, given good answers to the following:

• what is the valid placement and effective scope of in-file options - do they affect the entire file they're in, the rest of the file, included files, subsequent files, all files... ?
• in which file formats should they be supported ?
• short options with one hyphen and long options with two hyphens should work. Maybe also two hyphens for short options, which current Ledger requires.
• in-file options should be overrideable by command-line options and arguments.