New program that prints cross entropy of files vs model

text-cross-entropy predicts the contents files named on the command line
and prints the corresponding cross entropy. Typical usage:

    find [...] | xargs ./text-cross-entropy -f some.net -m 200 -i 10

which will look at files longer than 200 bytes, ignoring the entropy
of the first 10 letters, and print something like this:

    a.txt 2.727
    b.txt 1.8914
    [...]

The `-p` option pretends that each file begins with the given string.
Combined with `-i` which ignores the start, this allows the net to smooth
over the sudden shock of starting from an empty state.
gitignore new executables (text-confabulate, text-cross-entropy)
charmodel rnn_char_load_metadata: sscanf fails on empty fields

With sscanf, when "collapse_chars" is empty, the next line gets consumed,
which breaks everything.

This redoes it with strsep, and probably gives more noticeable messages
of despair in case of problems.
charmodel: validation and text-cross-entropy share code

Both now use doubles for accumulating cross-entropy, so that much
larger texts can be accurately measured (the cumulative sum can make the
difference between the sum and each character's entropy large).