Allow to pass custom Cldr module

[dolfinus]

If one uses any language defined in Linguist.Cldr, it's fine, linguist will handle translations as usual.
If one uses any language which is unknown for Linguist.Cldr, but does not use pluralized keys, this is also fine - such operations as key search and string interpolation are also handled as usual.

But if one uses unknown language and a path which should be pluralized by Cldr, the error will be raised:

     ** (Protocol.UndefinedError) protocol String.Chars not implemented for {:error, {Cldr.UnknownLocaleError, "The locale \"ru\" is not known."}} of type Tuple. This protocol is implemented for the following type(s): Decimal, Date, URI, Time, Integer, Version.Requirement, DateTime, NaiveDateTime, Float, Atom, Version, BitString, List
     code: I18n.t("ru", "countable", count: 1)
     stacktrace:
       (elixir 1.11.0) lib/string/chars.ex:3: String.Chars.impl_for!/1
       (elixir 1.11.0) lib/string/chars.ex:22: String.Chars.to_string/1

This could be more annoying after #32, because fr and es locales will be removed from package default config.

In this PR I've added feature of passing custom Cldr backend module into linguist app.
Possible ways are described in README.md, there are plenty of them.

Also t() function will return {:error, _} without throwing any exception and t!() function will throw an exception which was returned from Cldr module instead of Protocol.UndefinedError.

WIP status is set because this fix depends on #31, which should be merged first.

[dolfinus]

@mertonium Could you please take a look at this?

[dolfinus]

@mertonium I've rebased commits to current master state to run tests. What do you think about this PR?