[feature] Hyperlinks to URLs

[kvid]

@martonmiklos has created PR #164 to remove HTML links from the input attributes. I like his idea, but I also want to discuss hyperlinks to URLs in general.

The user might want to add hyperlinks to several kinds of external references when documenting a harness:

• Other documents in the same project.
• Entries in a part number system.
• Product pages or main page of manufacturer, supplier, etc.

The different output formats can support hyperlinks in different ways:

• svg: Each node, edge and cluster can be attached to different URLs: https://www.graphviz.org/doc/info/attrs.html#d:URL
• png: Does not support hyperlinks directly, but together with a cmapx output it is possible: https://www.graphviz.org/doc/info/output.html#d:cmapx
• html: Support multiple hyperlinks anywhere, e.g. in BOM entries as in PR Remove HTML links from the input attributes  #164 and with any new harness attributes for HTML contents as in [feature] Accept optional (and overridable) harness attribute(s) #158 (comment).
• tsv: A new optional URL column can contain the literal URLs.

It might be possible to specify URLs in at least two ways in the YAML input:

• As HTML <a href="URL">-tags in text attributes, as in PR Remove HTML links from the input attributes  #164.
• As a new URL or href attribute to connectors, cables, additional_bom_items, and any new entries like subgraphs ([feature] Add heatshrink / sheathing information #31), components ([feature] Enable including components in the harness #142), and glands ([feature]Cable glands support? #155).

I see no major conflicts between the two alternatives above. The first one supports more than one hyperlink per BOM entry and custom link texts, The second one supports more output formats, and both can be allowed for full flexibility. I therefore suggest accepting PR #164 when it is ready (supporting hyperlinks in the HTML BOM). A new PR can be created to add attributes and supporting more output formats, like #168.