feat(#23): support optional KaTeX config file

Load settings from an optional `~/.vivify/katex_config.json` file to
apply [supported KaTeX options](https://katex.org/docs/options.html).

README.md

@@ -15,6 +15,12 @@ Vivify brings your (Markdown) files to life in the browser!
   support](#editor-support))
 - Vivify server starts lazily and automatically shuts down when no more viewers
   are connected
+- customize KaTeX to your needs by providing available [KaTeX
+  options](#KaTeX-options)
+
+If you need any additional features, feel free to [open an
+issue](https://github.com/jannis-baum/vivify/issues/new/choose) or
+[contribute](CONTRIBUTING.md)!
 
 ### Markdown
 
@@ -25,9 +31,19 @@ Vivify brings your (Markdown) files to life in the browser!
 - syntax highlighting for code
 - graphviz/dot graphs
 
-If you need any additional features, feel free to [open an
-issue](https://github.com/jannis-baum/vivify/issues/new/choose) or
-[contribute](CONTRIBUTING.md)!
+### KaTeX options
+
+Customize KaTeX to your needs by providing an optional
+`~/.vivify/katex_config.json` config file with [available KaTeX
+options](https://katex.org/docs/options.html). For example:
+```
+{
+        "errorColor": "#cc0000",
+        "macros": {
+                "\\RR": "\\mathbb{R}"
+        }
+}
+```
 
 ## Usage
 

src/parser/parser.ts

@@ -1,10 +1,17 @@
 import { homedir } from 'os';
+import fs from 'fs';
 
 import MarkdownIt from 'markdown-it';
 import anchor from 'markdown-it-anchor';
 import highlight from './highlight';
 import graphviz from './dot';
 
+const katexConfigPath = `${homedir()}/.vivify/katex_config.json`;
+let katexConfig = {};
+if (fs.existsSync(katexConfigPath)) {
+    katexConfig = JSON.parse(fs.readFileSync(katexConfigPath, 'utf8'));
+}
+
 const mdit = new MarkdownIt({
     html: true,
     highlight: highlight,
@@ -22,7 +29,7 @@ mdit.use(require('markdown-it-inject-linenumbers'));
 mdit.use(require('markdown-it-texmath'), {
 	engine: require('katex'),
 	delimiters: 'dollars',
-	katexOptions: { errorColor: '#cc0000', macros: { '\\RR': '\\mathbb{R}' } }
+	katexOptions: katexConfig
 });
 /* eslint-enable @typescript-eslint/no-var-requires */
 mdit.use(graphviz);