Permalinks should never try to create a folder outside(above) of Metalsmith(__dirname)

[nikmartin]

Related to #25:

.use(permalinks({
    relative: false,
    pattern: ':collection/:title'
  }))

will try to create about.md, which is NOT in a collection, as /about/index.html, which is the root of the drive on a LInux or OS X system. changing it to:

.use(permalinks({
    relative: false,
    pattern: './:collection/:title'
  }))

makes it relative to something, which happens to be the root of the build dir. What should probably happen is permalinks should always build paths relative to the build dir.

[IanMercer]

It would be preferable if the plugin code actually prevented the accidental use of a / at the start of a pattern. After getting the path maybe something like:

  // Remove leading / to avoid escaping to the root folder!!
  if (path.indexOf('/') === 0){
        path = path.substring(1);
  }

[webketje]

Solved in v3.0.0: https://github.com/metalsmith/permalinks/blob/v3.0.0/src/index.js#L211