Add exclude: param to relativize_paths filter

[denisdefreyne]

Implements nanoc/features/issues/41.

Detailed description

This allows specifying a list of excludes in the relativize_paths filter, e.g.

# excludes /foo, /foo/bar, …
# does not exclude /fooooo, /qux/foo, /qux/foo/asdf, …
filter :relativize_paths, type: :html, exclude: '/foo'

# excludes /foo, /foo/asdf, /bar, /bar/asdf, …
# does not exclude /fooooo, /qux/foo, /qux/foo/asdf, /qux/bar, …
filter :relativize_paths, type: :html, exclude: ['/foo', '/bar']

# excludes /foo/bar, /bar, /archive, /blog/archive/something, …
filter :relativize_paths, type: :html, exclude: /ar/

It can be used in different ways:

• string vs. regexp: When given a regular expression, the path will not be relativized if the regular expression matches. When given a string, the path will not be relativized when the path starts with one or more components that matches the given string.

• single object vs. array: The parameter can be either a single string or regexp, or an array of them.

To do

• Exclude only when fully matching a path component
• Allow specifying string instead of an array of strings
• Allow specifying an (array of) regex instead of an (array of) strings

After merge, before release:

• Document

[DaveEveritt]

👍