add Publisher Metadata

[mcDevnagh]

I would like to add a publisher to generated EPUBs
see dc:publisher

[duyk471]

There isn't any convenient way to directly add the publisher's name atm. Hopefully someone will pick it up and add it. If you're familiar with Rust, here is the current Metadata struct:

#[derive(Debug)]
pub struct Metadata {
    pub title: String,
    pub author: Vec<String>,
    pub lang: String,
    pub direction: PageDirection,
    pub generator: String,
    pub toc_name: String,
    pub description: Vec<String>,
    pub subject: Vec<String>,
    pub license: Option<String>,
    pub date_published: Option<chrono::DateTime<chrono::Utc>>,
    pub date_modified: Option<chrono::DateTime<chrono::Utc>>,
    pub uuid: Option<uuid::Uuid>,
}

Here is what it's supposed to have in the content.opf file:

<dc:publisher>Publisher's Name</dc:publisher>

Here is the code snippet for that, in the epub.rs file:

pub fn metadata<S1, S2>(&mut self, key: S1, value: S2) -> Result<&mut Self>
where
S1: AsRef<str>,
S2: Into<String>,
{
match key.as_ref() {
    "publisher" => self.metadata.publisher = value.into(),

Also updating the templates/v3/content.opf file by adding the <dc:publisher>{{publisher}}</dc:publisher> line.

I'm too lazy to open a pull request so here it goes.