Change appropriate defvar -> defcustom with typechecking

[rnkn]

I've changed all the defvar variables to defcustom options that I figured were appropriate, and small additions to the docstrings for setting to nil to disable sounds and alerts.

The initial values for options that include directories may now look odd because they are expressions rather than files, and the typechecking asks for a file, but these will eval to files upon load.

[TatriX]

Looks good to me. Thank you!