Allow rules to override -O2 with their own compiler_flags. (#373)

* Allow rules to override -O2 with their own compiler_flags.

Currently, `-c opt` will turn on optimization for all Haskell
rules with no way to disable it.  Previously, it was possible
to do so with `compiler_flags = ["-O0"]`, but that behavior was
changed in #342:
https://github.com/tweag/rules_haskell/pull/342/files#diff-33a8c46955e2a9917090b5258346aee2L247

Some Haskell packages take a *lot* of memory and a lot longer to compile with
optimizations.  So it's useful for us to be able to control this tradeoff.

haskell/private/actions/compile.bzl

@@ -275,12 +275,13 @@ def _compilation_defaults(hs, cc, java, dep_info, srcs, import_dir_map, extra_sr
         ghc_args += unit_id_args
 
     args = hs.actions.args()
-    args.add(ghc_args)
 
-    # Compilation mode and explicit user flags
+    # Compilation mode.  Allow rule-supplied compiler flags to override it.
     if hs.mode == "opt":
         args.add("-O2")
 
+    args.add(ghc_args)
+
     args.add(["-static"])
 
     # NOTE We can't have profiling and dynamic code at the same time, see: