Handle file paths with whitespace

[jason0x43]

Before submitting a pull request, please make sure the following is done...

1. Fork the repo and create your branch from master.
2. If you've added code that should be tested, do a sanity check on vim8 and neovim
3. If you've changed APIs, update the README and documentation ./doc/prettier.txt

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Summary

Wrap the path used for --stdin-filepath in quotes. This allows prettier to work with files that have whitespace in their paths.

references #108

Test Plan

1. Edit a file in a location with whitespace in the path (e.g., /Users/jason/Library/Mobile Documents/foo.md).
2. Run :Prettier.
3. Verify that prettier runs without error.

[mitermayer]

Thank you! Will test this and merge it ASAP

[mitermayer]

Merging this once builds pass