Make readtable()'s "quotemark" argument a vector, remove "allowquotes"

Making "quotemark" a vector allows quite naturally to choose several
quoting characters (potentially useful for some weird files), and
disabling the use of quoting altogether by passing an empty vector.
This simple implementation does not take into account a possible
nesting of two different types of quote marks. This allows keeping
the performance impact low. Cf. issue #400.