Deprecate @postlight/mercury-parser in npmjs

[mcorrello]

Deprecate @postlight/mercury-parser in npmjs to point users to use the new @postlight/parser package.

Run something along the lines of:

npm deprecate @postlight/mercury-parser@"<= 2.2.1" "This project has been renamed to @postlight/parser. Install using @postlight/parser instead." 

Deprecating the old package name relates to #688, changing the project name from Mercury Parser to Postlight Parser.

Npm package to deprecate: Mercury Parser

[mcorrello]

Package has been deprecated in NPM and will notify when a user installs it