Sorted out mime initialization.

Every mime related function in main __init__.py has a flag check for the
check if initialization has already done. This is nonsense, since it
should be done implicitly early on the converter is starting.

This commit straight the things out, and initialization is done in cli
module.

Also, function guess_type was removed, since it's just a proxy for
mimetypes.guess_type function.

ebook_converter/ebooks/conversion/plumber.py

@@ -14,7 +14,7 @@ from ebook_converter.ebooks.conversion.preprocess import HTMLPreProcessor
 from ebook_converter.ptempfile import PersistentTemporaryDirectory
 from ebook_converter.utils.date import parse_date
 from ebook_converter.utils.zipfile import ZipFile
-from ebook_converter import extract, walk, filesystem_encoding, get_types_map
+from ebook_converter import extract, walk, filesystem_encoding
 from ebook_converter.constants_old import __version__
 
 
@@ -1010,7 +1010,6 @@ OptionRecommendation(name='search_replace',
             from ebook_converter.utils.fonts.scanner import font_scanner  # noqa
         import css_parser, logging
         css_parser.log.setLevel(logging.WARN)
-        get_types_map()  # Ensure the mimetypes module is intialized
 
         if self.opts.debug_pipeline is not None:
             self.opts.verbose = max(self.opts.verbose, 4)