add prefixes config settings

[robsonsobral]

Sample:

$config['snippetssync_snippet_prefix'] = 'sn-';
$config['snippetssync_global_variable_prefix'] = 'gv-';

[bjornbjorn]

I have now tested this and it's working perfectly. Updated the version number + changelog. Thanks!

[robsonsobral]

Thank you so much for the opportunity to help you!

May I suggest you to add information about the allowed characters on docs, too? They're alpha-numeric characters, underscores, and dashes.

Thank you!