implementation of RFC #266 (#587)

* Feature complete for #266 with exception of several small deviations:
1. initial image and model weight hashes use full sha256 hash rather than first 8 digits
2. Initialization parameters for post-processing steps not provided
3. Uses top-level "images" tags for both a single image and a grid of images. This change was suggested in a comment.

* Added scripts/sd_metadata.py to retrieve and print metadata from PNG files
* New ldm.dream.args.Args class is a namespace like object which holds all defaults and can be modified during exection to hold current settings.
* Modified dream.py and server.py to accommodate Args class.

scripts/sd-metadata.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python
+
+import sys
+import json
+from ldm.dream.pngwriter import retrieve_metadata
+
+if len(sys.argv) < 2:
+    print("Usage: file2prompt.py <file1.png> <file2.png> <file3.png>...")
+    print("This script opens up the indicated dream.py-generated PNG file(s) and prints out their metadata.")
+    exit(-1)
+
+filenames = sys.argv[1:]
+for f in filenames:
+    try:
+        metadata = retrieve_metadata(f)
+        print(f'{f}:\n',json.dumps(metadata, indent=4))
+    except FileNotFoundError:
+        sys.stderr.write(f'{f} not found\n')
+        continue
+    except PermissionError:
+        sys.stderr.write(f'{f} could not be opened due to inadequate permissions\n')
+        continue