Implemented dry run mode

Added a --dry-run flag that compiles and steps through each task, but does not execute them. The commands that would have been run are printed. See #125.

completion/zsh/_task

@@ -12,6 +12,7 @@ function __list() {
 
 _arguments \
     '(-d --dir)'{-d,--dir}': :_files' \
+    '(--dry-run)'--dry-run \
     '(-f --force)'{-f,--force} \
     '(-i --init)'{-i,--init} \
     '(-l --list)'{-l,--list} \