Add a command line argument to specify the base uri

[stevorobs3]

Closes #67

[RickMoynihan]

ok I can't quite see what this PR actually does, do you have an example of it having an effect? I think also when we're agreed on what it should do (see below) some tests would be good.

I think the main problem is that it probably wasn't very clear what #67 was asking for, as there are several contexts where base-uri might come in to play. Also I don't think this was fully appreciated when I wrote the issue (Feb 16 2022) as csvw has some highly confusing specified behaviour when it comes to base URI's:

https://github.com/Swirrl/csvw-issues/blob/main/issues/002-template-evaluation.md
w3c/csvw#888

Essentially the TLDR; of the above is that there are two base URI's, one used for the JSON-LD metadata document, and the other (the csv file url used for the expansion of uri templates in that same JSON-LD document).

I think this is wildly counter intuitive, and essentially all but one person in the working group seemed to think so too; yet there was a meeting and they decided for undocumented reasons to specify against the consensus (sensible?) option.

So I think what is most useful here is for -b to override the base-uri that is used for both URI template expansion and expanding relative URI terms in the JSON-LD metadata document, in our annotated mode.

The purpose is to support relative URI's better. The problem is that the CSVW files when "re-based" onto a different URI will yield different RDF to what the files without -b will. Which means you can't really publish those files; but I think that's fine, a common use case for csv2rdf is to convert csv files into triples, and to not publish the CSV or csvw-metadata file... i.e. the -b flag is really to support extra flexibility as a transformation tool.

I can imagine the possibility that there might be multiple base-uri flags e.g. --table-base-uri --jsonld-base-uri, which would allow you to set these two contexts independently, but I can't think of many situations where you'd want them to differ, so would prefer to set them both simultaneously for now.