Add Finnish locale

Tests copy-pasted from the `ca-ES` tests :)

For anyone wondering how to use the locales as of `v3.4.6` (49ba8afebb2ae813ab66dc2f48f533aa7f333c3c), I just patched [`src/format/format.js`](https://github.com/mbostock/d3/blob/master/src/format/format.js) and [`src/time/format.js`](https://github.com/mbostock/d3/blob/master/src/time/format.js):

```
~/d3 ❯❯❯ git diff src/format/format.js
diff --git a/src/format/format.js b/src/format/format.js
index bf740db..f103287 100644
--- a/src/format/format.js
+++ b/src/format/format.js
@@ -1,3 +1,3 @@
-import "../locale/en-US";
+import "../locale/fi-FI";

-d3.format = d3_locale_enUS.numberFormat;
+d3.format = d3.locale.fi_FI.numberFormat;
~/d3 ❯❯❯ git diff src/time/format.js
diff --git a/src/time/format.js b/src/time/format.js
index 3ad7f5f..e23fe63 100644
--- a/src/time/format.js
+++ b/src/time/format.js
@@ -1,4 +1,4 @@
-import "../locale/en-US";
+import "../locale/fi-FI";
 import "time";

-var d3_time_format = d3_time.format = d3_locale_enUS.timeFormat;
+var d3_time_format = d3_time.format = d3.locale.fi_FI.timeFormat;
```

As locale data is public domain, I doubt there's need for the Contributor License Agreement, right?