clean names of target markdown chunks

[jmbuhr]

Would it make sense to replace - in the names of target markdown chunks set up as tar_simple targets with _ when they become targets to turn them into valid R variable names?

[wlandau]

Unfortunately, underscores in chunk labels still interfere with bookdown: #473 (comment), rstudio/bookdown#1167. And I would prefer not all targets to use tar_simple = TRUE so that I can demonstrate the full range of functionality in Target Markdown.