Add default value macro

[jharrilim]

This PR adds an alternative macro, dotenv_or_default, which will provide a default value if the environment variable does not exist.

Example:

#[macro_use]
extern crate dotenv_codegen;

use actix_web::{
    App,
    HttpServer,
};

#[actix_rt::main]
pub async fn main() -> std::io::Result<()> {
  let hostname = dotenv_or_default!("HOSTNAME", "127.0.0.1");
  let port = dotenv_or_default!("PORT", "8080");
  let url = format!("{}:{}", hostname, port);
  let server = HttpServer::new(|| { App::new() })
    .bind(url)?
    .run()
    .await
}

[codecov]

Codecov Report

Merging #61 into master will decrease coverage by 5.06%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   86.77%   81.71%   -5.07%     
==========================================
  Files           7        7              
  Lines         242      257      +15     
==========================================
  Hits          210      210              
- Misses         32       47      +15     

Impacted Files	Coverage Δ
dotenv_codegen_implementation/src/lib.rs	0.00% <0.00%> (ø)
dotenv/src/find.rs	90.90% <0.00%> (+3.95%)	⬆️

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[ghost]

Was this file added intentionally? I can't see a place that it is used.

[jharrilim]

These stderr files are generated for the tests that use trybuild. It compares this to the stderr from compiling the associated rs file: https://github.com/dtolnay/trybuild#workflow

edit: This style of test is also used in the Rust project itself!
https://github.com/rust-lang/rust/tree/master/src/test/ui