add validate command that checks for portability of shared libraries

Plan on adding node-pre-gyp validate command.

We can programmatically query the built binary module, scan for dynamic shared library dependencies, and do various diagnostics/sanity checks.

• OS X: otool -L (or https://github.com/nico/lssym)
• Linux: ldd or readelf -d
• Windows: depends.exe (https://github.com/mapbox/mapbox-studio/blob/mb-pages/test/check_shared_libs.py#L38-L57)

At the least the command should print some diagnostics for the developer to inspect:

• What shared libraries are depended upon (and what do they depend on)
• On OSX / Linux do any shared libraries occur with absolute rpaths in locations like /usr/local/lib (which would indicate linking happened to non-portable library)

Start with the planned node-pre-gyp validate command and review the platform tools named in the issue: otool or lssym, ldd or readelf, and depends.exe. Done means the command reports shared-library dependency chains and flags non-portable absolute rpaths on OS X and Linux.