Add command line flag to provide the path to the output epub file

[szabgab]

e.g. --epub path/to/book.epub as in:

mdbook-epub -s true  ../book-source/ --epub path/to/book.epub

IMHO It would be easier than changing the book.toml file and also it would allow putting the output file in a totally different place.

[blandger]

Do we need separate 'output path' and 'file name' params?
Or better make one param like 'output path with file name' instead?

[NuLL3rr0r]

IMHO, separation might give the user more control. Sometimes you'd like to just specify the output path and do not care about the file name. Sometimes you're fine with the default path and just want to change the filename. Or, if you feel the need both.