add alias exts in COMPILERS for "translated" content without a def. lang post

when somebody had the old TRANSLATIONS_PATTERN="{path}.{ext}.{lang}"
DEFAULT_LANG="en"
and a post "something.rst.es" and no "something.rst", the former
would be considered a file with wrong extension
this commit adds aliases like ".rst.es" to the COMPILERS at runtime
move extension lang-alias creation code to Nikola.__init__

to create the aliases only once and not in each function call