add a couple of missing quotation marks
replace backticks by $() notation throughout the code

Let's keep this consistent
introduce SNOTES_OPTION_CHAR='!'

Ok, ' ' restricts the user, '/' collides with standard unix path semantics...
I hereby change it to '!' and turn it into a configuration variable.

Might be overkill, but as I've changed it twice already and
it occurs in more than one line in the source, it makes sense.
remove harmful backslashes
add more missing quotation marks
add partial support for subfolders

Given the user manually creates the respective folder
in the notes database, he can now add notes to that folder
and they are properly listed
don't allow '.' in note name - replace by '_'

This is to make sure something like "note:../notename" does not get through