Add constructor which allows specification of base directory

[markashleybell]

If the same ignore list is used at various levels (e.g. solution root and project roots), relative rules do not get applied correctly in child paths (because rules are assumed to be relative to the overall root.

If we allowed a base directory to be passed in, we could dynamically replace the base path in any rules in the list with a forward slash, which would mean the list still worked correctly from within the base path.

[Smaug123]

@nojaf Fixing this issue would fix the bug we found in Fantomas. It's ultimately thanks to the TrimStart on https://github.com/markashleybell/MAB.DotIgnore/blob/master/MAB.DotIgnore/IgnoreRule.cs#L171 .