Now feeds all data to uchardet, when uchardet is available. The file
size limit is removed.
When uchardet is not available, we look for UTF-8 BOM.
This should make loading UTF-8-BOM files faster.
Because Aegisub always save file in UTF-8-BOM, this should also
guarentee Aegisub will load large (>100MB) file saved by itself.
See Aegisub/Aegisub#110