added install instructions for package.el
Merge branch 'master' into dev

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reorganizing code
added TODO
update to be compatible with Stan Language v1.3.0

- added new variable: stan-rng-list
- replaced variable: stan-c++-keywords with stan-reserved-list
- updated highlighting syntax for Stan Language v. 1.3.0
fix version number in header
update snippets

- added unit_vector snippet
- renamed directory from bounds to range_constraints
- bind all range constraint snippets to key "<"
- added range_constraints to matrix, vector, and row_vector snippets
update versions to 1.2.0