Enable theming

[joseluis]

Currently it doesn't seem to be possible to configure the theme. E.g.:

/// ```mermaid
/// %%{init: {'theme':'base'}}%%
/// graph LR
///     s([Source]) --> a[[aquamarine]]
///     r[[rustdoc]] --> f([Docs w/ Mermaid!])
///     subgraph rustc[Rust Compiler]
///     a -. inject mermaid.js .-> r
///     end
/// ```
pub fn example() {}