At the moment, using the EmailTemplate() doesn't allow Metadata param…

…eters.

EmailTemplate() now accept Metadata parameters.

e.g.
```
postmark.emails.EmailTemplate(
	TemplateId=template_id,
	TemplateAlias=template_alias,
	From=f"{from_name} {from_}",
	To=user.email,
	Headers={"X-Accept-Language": "en-us, en"},
	TemplateModel=template_model,
	Metadata=metadata
)
```

postmarker/models/emails.py

@@ -380,6 +380,7 @@ def send_with_template(
         TrackLinks="None",
         Attachments=None,
         InlineCss=True,
+        Metadata=None,
     ):
         return self.EmailTemplate(
             TemplateId=TemplateId,
@@ -397,6 +398,7 @@ def send_with_template(
             TrackLinks=TrackLinks,
             Attachments=Attachments,
             InlineCss=InlineCss,
+            Metadata=Metadata
         ).send()
 
     def send_batch(self, *emails, **extra):
@@ -467,6 +469,7 @@ def EmailTemplate(
         TrackLinks="None",
         Attachments=None,
         InlineCss=True,
+        Metadata=None,
     ):
         """
         Constructs :py:class:`EmailTemplate` instance.
@@ -490,6 +493,7 @@ def EmailTemplate(
             TrackLinks=TrackLinks,
             Attachments=Attachments,
             InlineCss=InlineCss,
+            Metadata=Metadata,
         )
 
     def EmailBatch(self, *emails):