Add "with-axum" feature to allow compilation when other dependencies use axum

[dureuill]

Hello,

thanks for developing plotly ❤️

I'm currently trying to use it for a server backend based on axum that also uses askama for templates, and unfortunately that combination results in a compilation error as we run afoul of https://github.com/djc/askama/issues/810:

Compiler errors

error[E0433]: failed to resolve: could not find `askama_axum` in the list of imported crates
  --> /Users/dureuill/.cargo/git/checkouts/plotly-2d2da452b8500b54/0351510/plotly/src/plot.rs:14:10
   |
[package]
14 | #[derive(Template)]
   |          ^^^^^^^^ could not find `askama_axum` in the list of imported crates
   |
   = note: this error originates in the derive macro `Template` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: could not find `askama_axum` in the list of imported crates
  --> /Users/dureuill/.cargo/git/checkouts/plotly-2d2da452b8500b54/0351510/plotly/src/plot.rs:21:10
   |
21 | #[derive(Template)]
   |          ^^^^^^^^ could not find `askama_axum` in the list of imported crates
   |
   = note: this error originates in the derive macro `Template` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: could not find `askama_axum` in the list of imported crates
  --> /Users/dureuill/.cargo/git/checkouts/plotly-2d2da452b8500b54/0351510/plotly/src/plot.rs:31:10
   |
31 | #[derive(Template)]
   |          ^^^^^^^^ could not find `askama_axum` in the list of imported crates
   |
   = note: this error originates in the derive macro `Template` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: could not find `askama_axum` in the list of imported crates
  --> /Users/dureuill/.cargo/git/checkouts/plotly-2d2da452b8500b54/0351510/plotly/src/plot.rs:38:10
   |
38 | #[derive(Template)]
   |          ^^^^^^^^ could not find `askama_axum` in the list of imported crates
   |
   = note: this error originates in the derive macro `Template` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0433`.
error: could not compile `plotly` (lib) due to 4 previous errors

To fix these issues, I added the with-axum feature that adds the askama_axum additional dependency when enabled (as well as the with-axum of the askama crate).

Users encountering the compilation errors above need to enable the with-axum feature of plotly to fix the errors. I tested locally and it enables compilation again 👍

Thanks for reading 🚀

[andrei-ng]

@dureuill , thanks for your PR. Looks good. Will try to get this merged in the next weeks.

[andrei-ng]

Maybe the feature name could be a bit better, but naming is always hard 😁
Will merge it like this for now.