Add some windows-* aliases to supported encodings list for detection

Why?
windows-1252 is an alias for ISO-8859-1 but not returned as an entry
in the supported encodings list for the detection API. This means the
lookup table we build for Ruby-compatible encoding names won't include
it and lookups for that encoding name will fail.

From the ICU documentation:
Note: Multiple different charset encodings in a same family may use a single shared name in this implementation. For example, this method returns an array including "ISO-8859-1" (ISO Latin 1), but not including "windows-1252" (Windows Latin 1). However, actual detection result could be "windows-1252" when the input data matches Latin 1 code points with any points only available in "windows-1252".
Merge pull request #69 from brianmario/add-windows-aliases

Add some windows-* aliases to supported encodings list for detection