Add `:only` dep option to filter on environments

mix deps.get will by default fetch all deps unless --only flag is given.
Merge pull request #2079 from ericmj/mix-dep-only

Add `:only` dep option to filter on environments
Fixing doc typos in Kernel
Merge pull request #2085 from jwarwick/special_typo

Fixing doc typos in Kernel
Improve error message for illegal use of variable in map key, closes #2083
Ensure struct fields are properly escaped when injected into AST