format-table: add new string cell type that accepts ANSI sequences

format-table: add new string cell type that accepts ANSI sequences

For various usecases it's useful that we can embed ANSI sequences in
cells of tables. For example, I hope we can eventually switch "systemctl
status" output to use the table formatter, and multiple of its fields
contain ANSI sequences (since they pack multiple different pieces
information into the same field, and highlight parts of it to
communicate relevance of distinct parts).

Add a distinct cell type for this, which gets special processing when we
output to a terminal that doesn't support ANSI sequences, and to JSON:
we strip the sequences.