Modified the way the command string is constructed for calling the text-...

...based browsers (w3m, lynx, etc.).
this fixes the previously existing limitation that it was not possible to pass any arguments to the selected text-based browser by defining an entry in `a2x.conf' such as

LYNX = "elinks -dump-width 80"

moreover, the old call for `w3m' (the default) _did_ explicitly set  a (rather small) width. I removed this setting so that behaviour is more consistent (specify width always explicitly, if desired, for the selected browser (defined via W3M or LYNX) or accept the default width used by that browser.