Support Windows-style container.xml file path and allow chapter failed to be parsed

[dothanhtrung]

Hi, I have an epub file which can be opened in other application like FBReader, but bk crashed when open it.

(You can get the epub file at: http://www.mediafire.com/download/ff5jqt6w83v1dca/Thon+Phe+Tinh+Khong+-+Nga+Cat+Tay+Hong+Thi.epub)

$ bk Thon\ Phe\ Tinh\ Khong\ -\ Nga\ Cat\ Tay\ Hong\ Thi.epub 
thread 'main' panicked at src/epub.rs:49:14:
called `Result::unwrap()` on an `Err` value: FileNotFound
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Reason:

1. This epub file has Windows-style path: META-INF\container.xml instead of META-INF/container.xml.
2. Some chapters are failed to be parsed. Issue may come from roxmltree.

Fix:

1. Try to read META-INF\container.xml if file META-INF/container.xml does not exist.
2. Although issue may come from romlxtree, bk can just simply ignore fault chapter, and continue to parse other chapters.