Fix relative path issue #293

Added option 'basepath' so that long filenames in the reports can be avoided. For example, by passing '/foo/bar/' as basepath, paths like '/foo/bar/src/index.js' will be transformed into 'src/index.js'.
I think this causes some reporters to fail (i.e., html-cov in mocha), since they need to look up the original file.  Should there be a third option for relative path from cwd?  @alex-seville @reimund

Example:
```
Error: BLANKET-Error parsing instrumented code: Error: Cannot find module '//./author'
    at /Users/.../Projects/factory/node_modules/blanket/src/index.js:167:35
```