PR #109 and issue #106

For PR #109 (incorrect wrapping of East Asian double-width chars in tables), adds an is_width arg to safe_substr() to make it interpret the length arg as a spacing width, and uses the East Asian Width regex (now refactored to load in a function) to adjust accordingly (a slow algorithm but couldn't think of anything better). Uses this in Ascii::row().

For issue #106 (related wp-cli/wp-cli#2458), adds pre_colorized arg to various functions, and adds Ascii::set/isPreColorized() and Table::set/isAsciiPreColorize() (the latter to be used via a corresponding ascii_pre_colorized arg to be added to WP_CLI/Formatter, see gitlost/wp-cli#15.)

Also centralizes decolorization into Colors::decolorize(), adding a keep arg to optionally keep either tokens or encodings.

Also makes sure Colors::colorize() only reads from cache after checking shouldColorize().

Also marks as unused the colored arg to Colors::cacheString(), and stops using the decolorized cache element in length() and width() as it never gets looked up if passed a colorized string (as they're indexed on the original string).

Also replicates the padding logic of safe_str_pad() in Colors::pad() and moves decolorization to Colors::width(), as it seems better that low-level funcs don't reference Colors.

Also makes length arg of safe_substr() cross PHP compat whether null or false.

Also adds an optional encoding arg to various functions to save some processing, and makes mb_detect_encoding() use strict.