Support ttf fonts

GNU Unifont has a full-featured TrueFont http://unifoundry.com/unifont/index.html

But right now bracket-lib cannot support ttf fonts.

For example, in `unicode.rs`:

```rust
bracket_terminal::embedded_resource!(TILE_FONT3, "../resources/unifont-13.0.03.ttf");

fn main() -> BError {
bracket_terminal::link_resource!(TILE_FONT3, "resources/unifont-13.0.03.ttf");

let context = BTermBuilder::new()
.with_dimensions(80, 50)
.with_tile_dimensions(16, 16)
.with_title("Hello Minimal Bracket World")
.with_font("unifont-13.0.03.ttf", 16, 16)
.with_simple_console(80, 50, "unifont-13.0.03.ttf")
.build()?;

let gs: State = State {};

main_loop(context, gs)
}

```

Will throw errors.

BTW, if I use the BMP version: http://unifoundry.com/pub/unifont/unifont-13.0.03/unifont-13.0.03.bmp

The loading time will be incredibly slow (more than 1min I think)