add option to support Mithril.js

Now I edit the source of pug-as-jsx-utils and change in the index.js line 167
result.useFragment ? "import React, { Fragment } from 'react';" : "import React from 'react';",
to
result.useFragment ? "import m, { Fragment } from 'mithril';" : "import m from 'mithril';",
it can work, but I think it's better to expose an option, like
'@babel/plugin-transform-react-jsx', {"pragma": "m"}
it's more convenient and useful. May consider.

@Onesimu
Added pragma: 'preact' | 'mithril' option. Please upgrade to version 1.0.72, try it out and let me know if it works.

pug-as-jsx-loader/test/issues.js

Lines 39 to 48 in 14259e7

    
           `import m from 'mithril'; 
        
           export default function() { 
        
             return ( 
        
               <div> 
        
                 <h4>hello world</h4> 
        
               </div> 
        
             ); 
        
           } 
        
           `, { pragma: 'mithril' }],

	`import m from 'mithril';

	export default function() {
	return (
	<div>
	<h4>hello world</h4>
	</div>
	);
	}
	`, { pragma: 'mithril' }],