feat(bkgpslog):Add functions, group code into functions

Add several fucntions to add future new feature (recipient dir watch)
as well as to group existing code blocks into their own functions for
readability. Changes include:

* Add checkAgePubkey() function: checks that a provided string doesn't
throw an error when fed as a recipient string to `age`.

* Pushed several code blocks into their own "magic" functions
** magicParseRecipientDir() : (future feature): To be run at each
buffer session to update the recipient array in response to changes in
a specified recipient directory.
** magicParseRecipientArgs(): Parses strings provided by '-r'
option and format output accordingly. Existing code.
** magicParseCompressionArgs(): Responds to '-c' option by specifying
compression function and output file suffix.
* Update showUsage() to document future '-R' recipient dir option.