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[bot]]

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%)	:arrow_up:

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