Use $FORM for the default args of FormLink()

When `args` is not specified, FormLink() (so form.open()) refers to
the environment variable $FORM. If unset, 'form' will be used.

Author: tueda

form/__init__.py

@@ -31,7 +31,8 @@ def open(args=None, keep_log=False):
 
     The optional argument `args` is for the FORM command, a string or
     a sequence of strings. For example '/path/to/form' or ['tform', '-w4'].
-    The default value is 'form'.
+    By default, the value of the environment variable `$FORM` is used if set,
+    otherwise 'form' will be used.
 
     The other argument `keep_log` indicates whether the log from FORM is kept
     and used as detailed information when an error occurs. If the value

form/formlink.py

@@ -106,7 +106,8 @@ def open(self, args=None, keep_log=False):
 
         The optional argument `args` is for the FORM command, a string or
         a sequence of strings. For example '/path/to/form' or ['tform', '-w4'].
-        The default value is 'form'.
+        By default, the value of the environment variable `$FORM` is used if
+        set, otherwise 'form' will be used.
 
         The other argument `keep_log` indicates whether the log from FORM is
         kept and used as detailed information when an error occurs.
@@ -120,7 +121,10 @@ def open(self, args=None, keep_log=False):
         FORM.
         """
         if args is None:
-            args = 'form'
+            if 'FORM' in os.environ:
+                args = os.environ['FORM']
+            else:
+                args = 'form'
 
         if isinstance(args, string_types):
             args = shlex.split(args)  # Split the arguments.