Add path resolution helpers

[SBoudrias]

Using the new file system, I opted to not automatically resolves path. I think experiences this year proved me it's better to be a bit more descriptive than hiding too much logic behind the scene.

Although this doesn't mean we should be super verbose all the time:

this.fs.copy(
  path.join(this.sourceRoot(), 'fileA.js'),
  path.join(this.destinationRoot(), 'fileB.js'),
)

So we want helper methods to join paths with source, destination, cache (and remote or are they the same?).

I'm also unsure the vocabulary we use is the best. "source" is actually the template/ directory, and destination is the project folder. So, should we be more semantic in the way we name them?

this.templatePath('/fileA.js');
this.projectPath('fileB.js');

I'd like your opinion on that!

[sindresorhus]

this.fs.copy(
  [this.sourceRoot(), 'fileA.js'],
  [this.destinationRoot(), 'fileB.js']
)

Maybe accept an array in src/dest which are path.join'd?

[SBoudrias]

I'm not sure passing array would be very intuitive. I added this.templatePath() and this.destinationPath() helpers methods.