Add expansion of params in config file

Allow using wildcards (i.e. * to match any character and ~ to match the
user's home directory) in inputs and ouput variables in config file

simple_backup/simple_backup.conf

@@ -1,14 +1,14 @@
-#Example config file for simple_backup
+# Example config file for simple_backup
 
 [default]
-#Input directories. Use a comma to separate items
+# Files and directories to backup. Multiple items can be separated using a comma (','). It is possible to use wildcards (i.e. '*' to match multiple characters and '~' for the user's home directory).
 inputs=/home/my_home,/etc
 
-#Output directory
+# Output directory.
 backup_dir=/media/Backup
 
-#Exclude patterns. Use a comma to separate items
-exclude=.gvfs,.local/share/gvfs-metadata,.cache,.dbus,.Trash,.local/share/Trash,.macromedia,.adobe,.recently-used,.recently-used.xbel,.thumbnails
+# Files, directories and patterns to exclude from the backup. Multiple items can be separated using a comma.
+exclude=*.bak
 
-#Number of snapshots to keep (use -1 to keep all)
-keep=-1
+# Number of old backups (i.e. excluding the one that's being created) to keep (use -1 to keep all)
+keep=-1