Multiple font files in case of glyph not found

[arieClass]

Hey @vladimirgamalyan,
First of all, thank you for this wonderful bitmap font generator.
I wonder if there is a possibility to add the option, to give under --font-file more than one font, so in case that some chars aren't included in the font file, it will fall back to other font file or maybe a default font file like rubik.ttf or ariel.ttf.
Maybe add a new parameter to the command like --default-font-file which will be the fallback font in case of glyph not found.

[vladimirgamalyan]

Hi! Thank you! There was this feature before 8554a15 but I removed it as I couldn't figure out how to calculate correct font metrics fitting BMFont format specification