Add tests for clean method.

Thanks for explaining in #58 and #59 about `.clean()`, but it still doesn't seem to work as intended.

I added a test file to make sure `.clean()` works as intended, and to serve as documentation. I didn't use ranges this time so it should be correct.

Only 3/8 pass. Output:
````
ok 1 clean(1.2.3) = 1.2.3
not ok 2 clean( 1.2.3 ) = 1.2.3
not ok 3 clean( 1.2.3-4 ) = 1.2.3-4
not ok 4 clean( 1.2.3-pre ) = 1.2.3-pre
not ok 5 clean(  =v1.2.3   ) = 1.2.3
ok 6 clean(v1.2.3) = 1.2.3
not ok 7 clean( v1.2.3 ) = 1.2.3
not ok 8 clean(	1.2.3) = 1.2.3
````

Test 5, `  =v1.2.3   `, is from the readme.