Skip UTF-8 continuation bytes ((ch & 0xC0) == 0x80)

It's very unlikely that anyone would want to count substrings starting inside
characters.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Print UTF-8 characters as-is (i.e. don't escape them)

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Added the substring-tag-rfc822 program

This program is used for coloring HTML response documents before feeding them
to `substring-frequencies', to help it distinguish code from human readable
text.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Support the --color and --cover-threshold options

--color treats every other byte as a color for the following byte, allowing
characters to be treated differently depending on context.  For example, you
may want to treat HTML tags differently from other text within HTML documents.

--cover-threshold sets a lower bound for the incremental covered required to
accept a new substring into the covering set.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>