feat: kepub conversions

Working *.epub to *.kepub.epub file conversions when
using a Kobo reader. Updated docker file to include
`kepubify` to convert to kepub. If not available the
file is just sent without conversion.

convert/convert.go

@@ -0,0 +1,14 @@
+package convert
+
+const (
+	MOBI_MIME = "application/x-mobipocket-ebook"
+	EPUB_MIME = "application/epub+zip"
+)
+
+type Converter interface {
+	// Whether or not the converter is available
+	// Usually based on the availability of the underlying tool
+	Available() bool
+	// Convert the input file to the output file
+	Convert(input string, output string) error
+}