Document the resource-path option to atom executable
Clarify spec-directory documentation in atom executable
Set resource path based on spec directory when running atom core specs

Fixes #3872
Document ATOM_DEV_RESOURCE_PATH environment variable in `atom —help`
Fix typo in comment
Drop unnecessary encoding argument
Preserve alphabetical sort order for options
Add environment variable to the usage string instead of at end
Merge pull request #3883 from atom/ns-core-specs-from-anywhere

Allow core specs from command line in atom repository regardless of its location