Correct handling of ansi colour codes when nbconverting to latex

nbconvert leaves non-printing characters such as ^[[m in latex after nbconversion of notebooks with ansi colour codes (e.g., from ls).  Latex can't cope with these even with unicode packages correctly installed, see PR 4806.  This PR corrects this and interprets blinking ansi codes as bold.  It adds tests for the ansi reset code "\x1b[m" and bold characters to the unit tests.