Desktop, Cli: Improved bold formatting support in Enex import (#1708)

* Dekstop,CLI: enex_to_md: add html/md test file pairs

* one pair for basic text formatting tags: strong, b, i, em
* and one using span tags with inline styles for bold formatting

Note: The html files include the Evernote-typical "linebreak tags inside of separate <div> tags"
to represent empty lines!

* Desktop,Cli: enex_to_md: support bold in span tags using inline styles

* function isSpanWithStyle() checks if further processing of a span tag
  makes sense
* function isSpanStyleBold() checks if bold formatting via styles is
  used - a similar function could be written for each span-inline-style-format
  that should be supported

* Desktop,Cli: enex_to_md: fix saving span attrs in state object

pushing attributes of span tag to state object now
happens outside of isSpanWithStyle()