Support for emojis & justified text in templates

Added

• support for emojis! Me precisely unicode characters above 0xFFFF in general, thanks to @moe-25
• Template can now insert justified text
• get_scale_factor utility function to obtain FPDF.k without having to create a document
• convert_unit utility function to convert a number, x,y point, or list of x,y points from one unit to another unit

Changed

• fpdf.FPDF() constructor now accepts ints or floats as a unit, and raises a ValueError if an invalid unit is provided.

Fixed

• Template background property is now properly supported - #203
  ⚠️ Beware that its default value changed from 0 to 0xffffff, as a value of zero would render the background as black.
• Template.parse_csv: preserving numeric values when using CSV based templates - #205
• the code snippet to generate Code 39 barcodes in the documentation was missing the start & end * characters.
  This has been fixed, and a warning is now triggered by the FPDF.code39 method when those characters are missing.

Fixed

• Detect missing uni=True when loading cached fonts (page numbering was missing digits)