Add options for creating cwd and tmp directories

[tschaub]

By default, process.cwd() and os.tmpdir() are created for new mock filesystems. To change this behavior, createCwd and createTmp options can be passed.

E.g.

// mock the filesystem without creating process.cwd()
mock({'/one': 'file contents'}, {createCwd: false});

// mock the filesystem without creating os.tmpdir()
mock({file: 'contents'}, {createTmp: false});

Fixes #71.

[gustavnikolaj]

I tested this change with a branch on my project, and it works after adding the options argument to this function https://github.com/tschaub/mock-fs/blob/options/lib/index.js#L109 and passing it through to FileSystem.create. :-)

[tschaub]

it works after adding the options argument to this function

Yikes. Apologies for making it awkward to test. I meant to expose the options to these functions. fd9ab8a makes this actually useful.