Updated 'desc' snippet and a few new entries

#### Update

introduce separate _label_ stage in `desc` snippet before exit.  This makes the workflow: `desc` `TAB` _first item label_ `TAB` _first item text_

Subsequent items can be added with the new `itd` snippet: `itd` `TAB` _item label_ `TAB` _item text_

#### New snippets:

`sc` (small caps)
`align` (_align_ math env)
`alignstar` (_align*_ math env)
`itd` (labelled item for description env)