Improve extdiff configuration.
In addition to the old cmd.foo, opts.foo hgrc entries, allow a more simple
alias = command [opts]... form. For example:
[extdiff]
cdiff = colordiff -uprN
% init
% commit
adding base
% qnew mqbase
% qrefresh
% qdiff
diff -r 67e992f2c4f3 base
--- a/base
+++ b/base
@@ -1,1 +1,1 @@ base
-base
+patched
% qdiff dirname
diff -r 67e992f2c4f3 base
--- a/base
+++ b/base
@@ -1,1 +1,1 @@ base
-base
+patched