`BarChart` customization and ASCII support.

[Charanor]

Looking at the source code BarChart overrides its ProgressBar's AsciiBar property with a non-ASCII character: █. I suggest keeping it as ProgressBar's default (-) and also adding support to BarChart to change the bar character:

// Override all bars
new BarChart().Bar('+');
// Override bars separately
new BarChart().Bar(unicodeBar: '█', asciiBar: '+');