Custom markup tags

[gwyndaf]

As per user forum discussion

From a user point of view, an example might be defining <bv> for backing vocals and <hc> for highlighted chord, used as:

[C]Lead vocal [<hc>G</hc>]lyric <bv>yeah, yeah, yeah</bv>

(I don't format chords ad hoc within songs, so not exactly sure whether markup should be fall inside or outside [ ]).

Config could be as brief as:

"markup_styles" : {
  "bv" : "style=italic color=#444444",
  "hc" : "background=yellow size=larger"
  }

Rationale/issues:

• Essentially, we define the parameters to be added for a span tag, so there's no need to put span in the definition, but that gets added in translation.
• Closing the user-defined tag, e.g. </bv> automatically translates to </span>, so no need to define that.
• Could this be used to redefine existing convenience tags? Ideally, I think yes, but don't know what the implications are.
• Custom tags won't make sense to third-party software (e.g. MSP), but that can be avoided by re-generating more compatible (but less readable) ChordPro files.
• This could go within the pdf config section, or more general settings so it works with other output formats. I think the point above suggests is should be a general setting, so custom tags get expanded to full <span> tags for ChordPro output as well as PDF.
• There may be overlap with more general discussion on custom styles (Custom styles #404), which seems a longer-term topic, but might be a consideration.