Support reading the subject from a file. (#173)

Allows dynamic subjects to be generated at action runtime
(e.g., "Foobar version 1.2.3 is released", where 1.2.3
is not known at action config time).

Leverages the existing file:// support for body, but renames
getBody to getText, in keeping with this expanded role.

.github/workflows/test.yml

@@ -30,7 +30,7 @@ jobs:
               <p>Paragraph</p>
               </body>
               </html>
-          - subject: Plain body (Markdown)
+          - subject: file://testdata/subject.txt
             convert_markdown: true
             body: file://README.md
           - subject: HTML body (Markdown)