add demote command

.klyban.csv

@@ -1,4 +1,4 @@
-,Unnamed: 0.1,Unnamed: 0,STATUS,ID,TITLE,DETAILS,TAG,DEADLINE
+,Unnamed: 0.4,Unnamed: 0.3,Unnamed: 0.2,Unnamed: 0.1,Unnamed: 0,STATUS,ID,TITLE,DETAILS,TAG,DEADLINE
-0,0,0,DOING,1,print card,pretty print fixed-width cards given a content,klyban,
+0,0,0,0,0,0,DOING,1,print card,pretty print fixed-width cards given a content,klyban,
-1,1,1,DONE,2,print table,pretty print set of cards on each column,klyban,
+1,1,1,1,1,1,TODO,2,print table,pretty print set of cards on each column,klyban,
-2,2,2,TODO,3,nested prints,print cards within cards,klyban,
+2,2,2,2,2,2,TODO,3,nested prints,print cards within cards,klyban,

klyban.py

@@ -89,7 +89,7 @@ def show(context):
 def promote(context, id):
     """Upgrade the status of task `ID` to the next one.
 
-    As configured with --status-list."""
+    Use status configured with --status-list."""
 
     df = load_data(context)
 
@@ -112,6 +112,36 @@ def promote(context, id):
 
     context.invoke(show)
 
+@cli.command()
+@click.argument('ID')
+@click.pass_context
+def demote(context, id):
+    """Downgrade the status of task `ID` to the previous one.
+
+    Use status configured with --status-list."""
+
+    df = load_data(context)
+
+    row = df.loc[ df[context.obj['id_key']] == int(id) ]
+    if row.empty:
+        error("ID_NOT_FOUND", "{} = {} not found in `{}`".format(context.obj['id_key'], id, context.obj['input']))
+
+    i=0
+    for i in range(len(context.obj['status_list'])):
+        if row[context.obj['status_key']][1] == context.obj['status_list'][i]:
+            break
+        else:
+            i += 1
+    if i == 0:
+        error("UNKNOWN_STATUS", "Cannot demote task {}, already at the first status.".format(id))
+    else:
+        df.loc[df[context.obj['id_key']] == int(id), context.obj['status_key']] = context.obj['status_list'][i-1]
+
+    save_data(context, df)
+
+    context.invoke(show)
+
+
 
 @cli.command()
 @click.pass_context