Txt2Tags reader: Header is now parsed only if standalone flag is set
Txt2Tags Reader: Parse Meta information

The header is now parsed as meta information. The first line is the
`title`, the second is the `author` and third line is the `date`.
Txt2Tags Reader: Corrected formatting of %%mtime macro
Txt2Tags Reader: Fixed crash when reading from stdin
Merge pull request #1553 from mpickering/master

Txt2Tags reader: Header is now parsed only if standalone flag is set