use argparse to specify file(s) to generate template.hpp

```
$ python3 ../mros2/mros2_header_generator/templates_generator.py --app echoback_string --file app.cpp bbb.cpp -h
Generate template.hpp from mros2 app code file.
usage: templates_generator.py [-h] [--app APP] [--file [FILE [FILE ...]]]

Generate template.hpp from mros2 app code file.

optional arguments:
  -h, --help            show this help message and exit
  --app APP             application name (default: 'echoreply_string')
  --file [FILE [FILE ...]]
                        filename(s) of mros2 app code (default: 'app.cpp')
```

mros2_header_generator/templates_generator.py

@@ -1,12 +1,10 @@
 import os
 import sys
 import re
+import argparse
 from os import path
 from jinja2 import Environment, FileSystemLoader
 
-arg = sys.argv
-app = arg[1]
-
 includeFiles = []
 pubMsgTypes = []
 subMsgTypes = []
@@ -15,7 +13,21 @@ def toSnakeCase(string):
     return re.sub("(.[A-Z])",lambda x:x.group(1)[0] + "_" +x.group(1)[1],string).lower()
 
 def main():
-    with open(app + "/app.cpp", 'r') as m_f:
+    print('Generate template.hpp from mros2 app code file.')
+
+    parser = argparse.ArgumentParser(description='Generate template.hpp from mros2 app code file.')
+    parser.add_argument('--app', default='echoreply_string',
+                    help='application name (default: \'echoreply_string\')')
+    parser.add_argument('--file', nargs='*', type=str, default=['app.cpp'],
+                    help='filename(s) of mros2 app code (default: \'app.cpp\')')
+
+    args = parser.parse_args()
+    app = args.app
+    file = args.file
+
+    for f in file:
+        print('  Analyzing {}/{} file to generate...'.format(app, f))
+        with open(app + "/" + f, 'r') as m_f:
             arr = m_f.readlines()
             for m_line in arr:
                 if "create_publisher" in m_line:
@@ -44,5 +56,7 @@ def main():
     with open(os.path.join(app+"/templates.hpp"), "wb") as f:
         f.write(datatext.encode('utf-8'))
 
+    print('Generate {}/template.hpp done.'.format(app))
+
 if __name__ == "__main__":
     main()