Fix --flatten flag parsing

Lines starting with `--` are considered options in docopt, and
`--flatten` was present at the start of a description line as
well as listed in the options sectioon; this caused docopt to
record --flatten twice, giving the cryptic message:

`--flatten is not a unique prefix: --flatten, --flatten?`

sigh.

docopt does have some serious downsides.

README.md

@@ -45,8 +45,10 @@ Options:
   -d DICTPATH --dictionaries=DICTPATH  Specify DICTPATH
   -f --flatten                         Flattens entity hierarchies into a flat array; entities are
                                          outputted in reverse order of dependency, and linked by "$id"
-  -s --split-output                    Outputs entities into files, separated by declared type; implies
-                                         --flatten
+  -s --split-output                    Aggregates entities by type into separate files; DESTFILE
+                                         serves as the filename template, meaning each file has the
+                                         entity type appended to its basename (i.e. before the ".json"
+                                         extension, as in "entities-myType.json"). Implies --flatten.
 ```
 
 ### Input File Format

main.go

@@ -32,8 +32,10 @@ Options:
   -d DICTPATH --dictionaries=DICTPATH  Specify DICTPATH
   -f --flatten                         Flattens entity hierarchies into a flat array; entities are
                                          outputted in reverse order of dependency, and linked by "$id"
-  -s --split-output                    Outputs entities into files, separated by declared type; implies
-                                         --flatten
+  -s --split-output                    Aggregates entities by type into separate files; DESTFILE
+                                         serves as the filename template, meaning each file has the
+                                         entity type appended to its basename (i.e. before the ".json"
+                                         extension, as in "entities-myType.json"). Implies --flatten.
 
 `
 )