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/cli.py

@@ -11,6 +11,7 @@ from ebook_converter.utils.config import OptionParser
 from ebook_converter.utils.logging import Log
 from ebook_converter.customize.conversion import OptionRecommendation
 from ebook_converter import patheq
+from ebook_converter import init_mimetypes
 from ebook_converter.utils.localization import localize_user_manual_link
 
 
@@ -361,6 +362,7 @@ def read_sr_patterns(path, log=None):
 
 def main(args=sys.argv):
     log = Log()
+    init_mimetypes()
     parser, plumber = create_option_parser(args, log)
     opts, leftover_args = parser.parse_args(args)
     if len(leftover_args) > 3: