Error when diff path contains folders that does not exist

'looks-same' saves the diff file using fs.createWriteStream(), passing that path as a parameter. Now, [fs.createWriteStream()](https://github.com/gemini-testing/looks-same/blob/5406adfb1e9a7b8e61c3224addc536320cbb9897/lib/png/png.js#L49) fails when the directory the path is pointing to doesn't exist - and that's the problem.

the shown error:
> ENOENT: no such file or directory, open '...'

possible solutions:
1. using fsExtra.outputFile() which creates any missing directories. (best IMO)
2. Implementing the recursive mkdir ourselves

initially found [here](https://github.com/oblador/loki/issues/229)

Start in lib/png/png.js at line 49, where fs.createWriteStream() saves the diff file. Reproduce the case with a path whose parent folders do not exist, then update the save behavior so those directories are created and the diff is written successfully.

Written by the indexing model from the issue text.