table plugin, src/common: Un-escape '\t', '\n' and '\r' in column separators.

table plugin, src/common: Un-escape '\t', '\n' and '\r' in column separators.

For this purpose, the function strunescape() has been added to the "common"
module. Currently, any escape sequence that's not '\t', '\n' or '\r' will be
expanded to the literal escaped character.