Aegisub

Introduce a feature in TextFileReader that checks whether a file seems to be binary rather than text.
Use this to make the MicroDVD (.sub) format reader check that it isn't being fed something that might actually be VobSub (.sub+.idx) which makes Aegisub crash.
Hopefully this doesn't break anything else.
Fixes #1351.

Originally committed to SVN as r5773.