add support for Finnish (FI) (#170)

FI: Add support for Finnish

This is a squash of the following commits :

* The recommendation by Kotus was optional. The code didn't work anyway
because million is 10**6, not 10**9, and the recommendation only applies
to round millions, billions and so on where the multiplier is 10 or
greater.
* FI: currency: use inheritance, add FIM and USD
* FI: add some common currencies
* FI: add inflections
* FI: inflections: small fixes
* FI: fix ordinal 200
* FI: fix comitative
* FI: actually fix ordinal 200
* FI: refactoring
* FI: fix tens
* FI: fix plural for tens and higher
* FI: toinen --> kahdes for numbers > 200
* FI: use collection for options
* FI: add tests
* FI: add minced tests
* FI: trim tests
* FI: finishing touches
* FI: improve cardinal_float, ordinal, add tests
* FI: add documentation to README.rst

README.rst

@@ -71,6 +71,7 @@ Besides the numerical argument, there are two main optional arguments.
 * ``es_CO`` (Spanish - Colombia)
 * ``es_VE`` (Spanish - Venezuela)
 * ``eu`` (EURO)
+* ``fi`` (Finnish)
 * ``fr`` (French)
 * ``fr_CH`` (French - Switzerland)
 * ``fr_BE`` (French - Belgium)
@@ -105,6 +106,38 @@ Therefore, if you want to call ``num2words`` with a fallback, you can do::
 Additionally, some converters and languages support other optional arguments
 that are needed to make the converter useful in practice.
 
+**fi (Finnish)**
+
+**case:** one of the following: nominative, genitive, partitive, inessive,
+elative, illative, adessive, ablative, allative, essive, translative,
+instructive, abessive, comitative. Defaults to nominative::
+
+    >>> num2words(42, lang='fi')
+    neljäkymmentäkaksi
+    >>> num2words(42, lang='fi', case='genitive')
+    neljänkymmenenkahden
+    >>> num2words(42, lang='fi', case='comitative')
+    neljinekymmeninekaksine
+    >>> num2words(42, lang='fi', to='ordinal', case='comitative')
+    neljänsinekymmenensinetoisine
+
+**plural:** make the output words plural::
+
+    >>> num2words(42, lang='fi', plural=True)
+    neljätkymmenetkahdet
+    >>> num2words(42, lang='fi', case='essive', plural=True)
+    neljinäkymmeninäkaksina
+    >>> # same in plural
+    >>> num2words(42, lang='fi', case='comitative', plural=True)
+    neljinekymmeninekaksine
+
+**prefer:** which case marker to use when there are multiple options::
+
+    >>> num2words(8, lang='fi', case="genitive", plural=True)
+    kahdeksien
+    >>> num2words(8, lang='fi', case="genitive", plural=True, prefer=["ain"])
+    kahdeksain
+
 **ja (Japanese)**
 
 **reading:** whether or not to return the reading of the converted number.