Support different date formats in lip headers

The following format in the header front matter of a lip causes a Docusaurus build error, I think due to the plugin that renders the lips

`05-29-2020`

You have to use

`05/29/2020`

```
Error: Minified React error #31; visit https://reactjs.org/docs/error-decoder.html?invariant=31&args[]=Thu%20May%2028%202020%2017%3A00%3A00%20GMT-0700%20(Pacific%20Daylight%20Time)&args[]= for the full message or use the non-minified dev environment for full errors and additional helpful warnings.
(undefined) Error: Minified React error #31; visit https://reactjs.org/docs/error-decoder.html?invariant=31&args[]=Thu%20May%2028%202020%2017%3A00%3A00%20GMT-0700%20(Pacific%20Daylight%20Time)&args[]= for the full message or use the non-minified dev environment for full errors and additional helpful warnings.
at T (main:25138:94)
at V (main:25138:229)
at X (main:25139:321)
at toArray (main:25140:398)
at a.b.renderDOM (main:25216:460)
at a.b.render (main:25209:326)
at a.b.read (main:25208:18)
at Object.renderToString (main:25218:364)
at render (main:51820:252)
Error: Failed to compile with errors.
at /Users/joelm/dev/lip-primary/node_modules/@docusaurus/core/lib/commands/build.js:37:24
at finalCallback (/Users/joelm/dev/lip-primary/node_modules/webpack/lib/MultiCompiler.js:254:12)
at /Users/joelm/dev/lip-primary/node_modules/webpack/lib/MultiCompiler.js:277:6
at done (/Users/joelm/dev/lip-primary/node_modules/neo-async/async.js:2931:13)
at runCompilers (/Users/joelm/dev/lip-primary/node_modules/webpack/lib/MultiCompiler.js:181:48)
at /Users/joelm/dev/lip-primary/node_modules/webpack/lib/MultiCompiler.js:188:7
at /Users/joelm/dev/lip-primary/node_modules/webpack/lib/MultiCompiler.js:270:7
at finalCallback (/Users/joelm/dev/lip-primary/node_modules/webpack/lib/Compiler.js:257:39)
at /Users/joelm/dev/lip-primary/node_modules/webpack/lib/Compiler.js:273:13
at AsyncSeriesHook.eval [as callAsync] (eval at create (/Users/joelm/dev/lip-primary/node_modules/tapable/lib/HookCodeFactory.js:33:10), :40:1)
at AsyncSeriesHook.lazyCompileHook (/Users/joelm/dev/lip-primary/node_modules/tapable/lib/Hook.js:154:20)
at onCompiled (/Users/joelm/dev/lip-primary/node_modules/webpack/lib/Compiler.js:271:21)
at /Users/joelm/dev/lip-primary/node_modules/webpack/lib/Compiler.js:681:15
at AsyncSeriesHook.eval [as callAsync] (eval at create (/Users/joelm/dev/lip-primary/node_modules/tapable/lib/HookCodeFactory.js:33:10), :4:1)
at AsyncSeriesHook.lazyCompileHook (/Users/joelm/dev/lip-primary/node_modules/tapable/lib/Hook.js:154:20)
at /Users/joelm/dev/lip-primary/node_modules/webpack/lib/Compiler.js:678:31
error Command failed with exit code 1.
```

See https://github.com/libra/lip/pull/7 for an example.