Add Option For Custom Span Handling

[VictorAlbertos]

Original issue: ireward/compose-html#1

[VictorAlbertos]

@owenlejeune the organization in which the repo was hosted before got destroyed, please try this new one with the desired changes on top of the last version:

com.github.viluahealthcare.compose-html:1.0.3

[dcanonne]

Thank you for this PR, imo it is highly important, il allows me to handle dark/light theme changes by doing this

customSpannedHandler = { AnnotatedString(it.toString(), SpanStyle(color = defaultTextColor)) }

defaultTextColor being defined as a color from the theme.

While i'm at it, thank you for this amazing and lightweight library <3