Allow for serving GitHub pages from non root directory

[RedSparr0w]

Description:

Will allow for generating a URL to the correct preview when serving GitHub pages from any directory other than the root directory.

Adds a new optional param which will be removed from umbrella-dir to determine the correct path to the preview.

Related issues:

closes #42

Notes:

Let me know if you think there might be a better approach to this.

[rossjrw]

If $pagesbase is e.g. docs/ and $targetdir is e.g. project-docs/pr-preview/pr-1, this would result in project-pr-preview/pr-1 which is not the intended result. (Would $pagesbase containing a slash character interfere with the replacement? My bash-fu isn't strong enough to instinctively know)

I think it's reasonable to assume that situation won't happen, because if the user is specifying a base dir you'd expect their path to start with that base dir. But it would also be nice to permit formats like docs, docs/, ./docs/, maybe even /docs/. I figure there must be some robust way to remove a path section in bash other than string substitution.

[RedSparr0w]

I've updated it to make use of regex instead, will only replace if at the start of the string now.
Also made the end / optional to close off pages-base-dir.

umbrella-dir: ./docs/preview-docs
pages-base-dir: ./docs
# pagespath: preview-docs/pr-123

umbrella-dir: /docs/preview-docs
pages-base-dir: /docs
# pagespath: preview-docs/pr-123

umbrella-dir: docs/preview-docs
pages-base-dir: docs
# pagespath: preview-docs/pr-123

umbrella-dir: docs/preview-docs
pages-base-dir: docs/
# pagespath: preview-docs/pr-123

This does sort of depend on the user typing the beginning the same for umbrella-dir and pages-base-dir (both start with ./path, /path or path)

But I think majority of the time this would happen naturally anyway, but added it to the documentation as well.