simple-call-tree-get-tags --> simple-call-tree-get-attribs

Also add keybinding to simple-call-tree-set-todo

Altered items:
simple-call-tree-mode, simple-call-tree-analyze,
simple-call-tree-get-attribs
Use chars instead of strings for priority attribute

Also add checks for invalid args to simple-call-tree-set-attribute

Altered items:
simple-call-tree-set-attribute, simple-call-tree-up-priority,
simple-call-tree-down-priority
Bugfix: handle nil attributes in simple-call-tree-set-attribute

Also finish simple-call-tree-set-priority

Altered items:
simple-call-tree-set-attribute, simple-call-tree-set-priority, simple-call-tree-insert-item