FPDF.circle, support for named HTML colors & repeating table headers on each page

Added

• disable font caching when fpdf.FPDF constructor invoked with font_cache_dir=None, thanks to @moe-25 !
• FPDF.circle: new method added, thanks to @viraj-shah18 !
• HTMLMixin / HTML2FPDF: support setting HTML font colors by name and short hex codes
• FPDF.will_page_break
  utility method to let users know in advance when adding an elemnt will trigger a page break.
  This can be useful to repeat table headers on each page for exemple,
  cf. documentation on Tables.
• FPDF.set_link now support a new optional x parameter to set the horizontal position after following the link

Fixed

• fixed a bug when fpdf.Template was used to render QRCodes, due to a forced conversion to string (#175)