add dataspec html to visc_git_ignore

[valduran18]

Closes #254. Adding dataspec html to the list of files that VISC ignores in .gitignore

Checklist

• This PR includes unit tests
• This PR establishes a new function or updates parameters in an existing function
  • The roxygen skeleton for this function has been updated using devtools::document
• I have updated NEWS.md to describe the proposed changes

[valduran18]

hi @yungwenliu and @lemireg, we're thinking of adding .html file types onto the list of files to ignore in the use_visc_gitignore() function. Would this conflict with SRA analysis repo workflow (i.e., are SRAs currently versioning html files in git?)