add enhancements to CLI command-line completion and history

- Added support for pyreadline3 so that Window users can benefit.
- Added the !search command to search the history for a matching string:
~~~
!search puppies
[20] puppies at the food bowl -Ak_lms
[54] house overrun by hungry puppies -C20 -s100
~~~
- Added the !clear command to clear the in-memory and on-disk
  command history.

ldm/dream/readline.py

@@ -17,8 +17,11 @@ from ldm.dream.args import Args
 try:
     import readline
     readline_available = True
-except:
-    readline_available = False
+except (ImportError,ModuleNotFoundError):
+    try:
+        import pyreadline3
+    except (ImportError,ModuleNotFoundError):
+        readline_available = False
 
 IMG_EXTENSIONS     = ('.png','.jpg','.jpeg')
 COMMANDS = (
@@ -47,7 +50,7 @@ COMMANDS = (
     '--skip_normalize','-x',
     '--log_tokenization','-t',
     '--hires_fix',
-    '!fix','!fetch','!history',
+    '!fix','!fetch','!history','!search','!clear',
     )
 IMG_PATH_COMMANDS = (
     '--outdir[=\s]',
@@ -62,7 +65,7 @@ IMG_FILE_COMMANDS=(
     )
 path_regexp = '('+'|'.join(IMG_PATH_COMMANDS+IMG_FILE_COMMANDS) + ')\s*\S*$'
 
-class Completer:
+class Completer(object):
     def __init__(self, options):
         self.options     = sorted(options)
         self.seeds       = set()
@@ -111,6 +114,19 @@ class Completer:
         if not self.auto_history_active:
             readline.add_history(line)
 
+    def clear_history(self):
+        '''
+        Pass clear_history() thru to readline
+        '''
+        readline.clear_history()
+
+    def search_history(self,match:str):
+        '''
+        Like show_history() but only shows items that
+        contain the match string.
+        '''
+        self.show_history(match)
+
     def remove_history_item(self,pos):
         readline.remove_history_item(pos)
 
@@ -137,7 +153,7 @@ class Completer:
     def get_history_item(self,index):
         return readline.get_history_item(index)
 
-    def show_history(self):
+    def show_history(self,match=None):
         '''
         Print the session history using the pydoc pager
         '''
@@ -149,7 +165,10 @@ class Completer:
             return
         
         for i in range(0,h_len):
-            lines.append(f'[{i+1}] {self.get_history_item(i+1)}')
+            line = self.get_history_item(i+1)
+            if match and match not in line:
+                continue
+            lines.append(f'[{i+1}] {line}')
         pydoc.pager('\n'.join(lines))
 
     def set_line(self,line)->None:
@@ -235,6 +254,9 @@ class DummyCompleter(Completer):
     def add_history(self,line):
         self.history.append(line)
 
+    def clear_history(self):
+        self.history = list()
+
     def get_current_history_length(self):
         return len(self.history)