Further improvements to Unicode documentation

The main improvement here is to make the tab completion table more searchable by adding Unicode names to the table of tab completions and their associated Unicode code points. The table generator script now downloads the Unicode names and does a table join to generate a new column with names for each code point.
There is also some attempt to cross-reference the table from the "Interacting with Julia" section of the manual, and a note about how fonts can mess with the rendering of Unicode characters.