Support for AngelCode BMFont format

Hi,

This pull request adds an ability to export bitmap fonts to the original AngelCode BMFont file format. Please review and apply it if you wish to support this format or just want to make a BMFont killa;)

Since BMFont doesn't support multiple fonts per .fnt file, they are saved to separate .fnt files using a single shared texture image e.g. myfont_0.fnt, myfont_1.fnt, myfont_2.fnt, myfont.png.

TIA!