file name starting with an underscore, leads to a compile bug

[hylerrix]

Describe the bug

[Pagic] Plugin sidebar start
error: Uncaught TypeError: Cannot read property 'title' of undefined
        title: pagic.pagePropsMap[sidebarConfigItem].title,
                                                     ^
    at https://deno.land/x/pagic@v0.10.3/src/plugins/sidebar.tsx:62:54
    at Array.map (<anonymous>)
    at parseSidebarConfig (https://deno.land/x/pagic@v0.10.3/src/plugins/sidebar.tsx:59:24)
    at Object.fn (https://deno.land/x/pagic@v0.10.3/src/plugins/sidebar.tsx:42:33)
    at Pagic.runPlugins (https://deno.land/x/pagic@v0.10.3/src/Pagic.ts:374:20)
    at async Pagic.rebuild (https://deno.land/x/pagic@v0.10.3/src/Pagic.ts:227:5)
    at async Pagic.build (https://deno.land/x/pagic@v0.10.3/src/Pagic.ts:204:5)

To Reproduce
Steps to reproduce the behavior:

• Name a file starting with an underscore, such as _hello.md, can not compile success.

Screenshots
If applicable, add screenshots to help explain your problem.

Context

• Pagic version: v0.10.3
• Deno version: v1.4.6
• Browser version (optional):
• OS version: macOS Big Sur v11(beta, 20A5395g)

[xcatliu]

Markdown file starts with _ will be ignored when compiling.