One multiline string to rule them all.

Adds multiline strings to TOML with a way to escape whitespace for
writing long single line strings.

I like this approach because it combines multiline strings and long single line strings into one form. All of these ideas were mentioned in one form or another in #92, #163 and #164.

I chose `"""` because that seems to be what people converged on. The obvious down-side is that `"""` cannot be typed into a multiline string. This isn't a very high price to pay, but if we didn't want to, then I'd suggest Lua's multiline string syntax:

```lua
-- Regular multiline string.
s = [[hello world]]

-- Want to write `[[something]]`?
s = [=[ hello [[something]] world ]=]
s = [=====[ this works too ]=====]
```

Basically, the quoting mechanism builds in the ability to change the delimiters.