Add extra settings features (#5)

 - Max scrollback
 - Max history length
 - Colors
 - Directory
 - Relocate history to a `.clam.history` file

The settings files look like:
```lua
{
  dir = "",
  aliases = {
  	grep = "glep",
  },
  env = {
  	HOME = "/me",
  },
  colors = {
  	prompt = "magenta"
  },
  maxScrollback = 200,
  maxHistory = 1000,
}
```