breaking: add cwd property to complement root

[clarfonthey]

Problems:

• Absolute paths are always relative to the system root. So, for example, /file doesn't mean the file in options.root, but /file on the system.
• Paths aren't relative to individual files. For example, in folder/file.html and folder/child/file.html, with root set to folder, the path file.txt is always folder/file.txt instead of folder/child/file.txt for the second.

Solution:

• The existing options.root is only applied to absolute paths.
• Introduce a separate options.cwd parameter which corresponds to the former options.root.
• Fall back to using the from path from posthtml is cwd is unset, only defaulting to the current directory if that isn't set.

Notes:

• This is a breaking change, but I can't imagine a way of making this that wouldn't be breaking. We could make root really be cwd and give some other name for root, but this would be too confusing imho.

[Scrum]

@clarfonthey What is stopping you from passing in the process.cwd() parameter?

[clarfonthey]

@Scrum Defaulting to process.cwd is a rather minor part of this change; the main purpose is to differentiate /absolute/paths and relative/paths such that the former is always relative to options.root, whereas the latter is relative to a new option, options.cwd.

Compare this to the current implementation, where /absolute/paths correspond to absolute paths on the system executing the code (not very useful, unless I want to <include src="/etc/profile"> or something), and relative/paths are always treated like the /absolute/paths as mentioned above.

[jeremysawesome]

Seemingly related issue #94