Added valtypeignore option

This allows, for example, serializing of tables where you're only concerned with `number`, `string`, and `boolean` types

Usage:
```lua
local a = {
    x = 'hello',
    y = function() return 1 end,
    z = 10
}

local options = {
    valtypeignore = {
        ['function'] = true,
        ['number'] = true
    }
}

print(serpent.block(a, options))

-- output:
{
  x = "hello"
}
```