Add --version option to Python based tools.

Version file is generated at build time and stored as _version.py.
To support running uninstalled tools, a fallback version is included as
well. Example output of `--version` option:

```
$ for t in g-ir-{annotation-tool,scanner,doc-tool}; do $t --version; done
g-ir-annotation-tool 1.57.1
g-ir-scanner 1.57.1
g-ir-doc-tool 1.57.1
```

1 files changed, 4 insertions, 1 deletions

diff --git a/giscanner/docmain.py b/giscanner/docmain.py
index 3bea5421..53f5206c 100644
--- a/giscanner/docmain.py
+++ b/giscanner/docmain.py
@@ -24,8 +24,10 @@ from __future__ import print_function
 from __future__ import unicode_literals
 
 import os
+import sys
 import argparse
 
+import giscanner
 from .docwriter import DocWriter
 from .sectionparser import generate_sections_file, write_sections_file
 from .transformer import Transformer
@@ -33,7 +35,8 @@ from .transformer import Transformer
 
 def doc_main(args):
     parser = argparse.ArgumentParser()
-
+    parser.add_argument('--version', action='version',
+                      version='%(prog)s ' + giscanner.__version__)
     parser.add_argument("girfile")
     parser.add_argument("-o", "--output",
                       action="store", dest="output",