gcode_macro: Parse variable_X parameters using ast.literal_eval()

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/Config_Changes.md

@@ -6,6 +6,11 @@ All dates in this document are approximate.
 
 # Changes
 
+20190607: The "variable_X" parameters of gcode_macro (along with the
+VALUE parameter of SET_GCODE_VARIABLE) are now parsed as Python
+literals. If a value needs to be assigned a string then wrap the value
+in quotes so that it is evaluated as a string.
+
 20190606: The "samples", "samples_result", and "sample_retract_dist"
 config options have been moved to the "probe" config section. These
 options are no longer supported in the "delta_calibrate", "bed_tilt",

docs/G-Codes.md

@@ -182,7 +182,8 @@ The following command is available when a "gcode_macro" config section
 is enabled:
 - `SET_GCODE_VARIABLE MACRO=<macro_name> VARIABLE=<name>
   VALUE=<value>`: This command allows one to change the value of a
-  gcode_macro variable at run-time.
+  gcode_macro variable at run-time. The provided VALUE is parsed as a
+  Python literal.
 
 ## Custom Pin Commands