Some names with underscore are not searchables and display without `_` in sidebar

[nelson6e65]

Scopes

• Output HTML
• Output Markdown
• Appearance
• Other

Describe the bug
Names with underscores are misinterpreted as markdown cursive and ignored by sidebars/search box

Steps to reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior

• Display the exact name in sidebar and searchbox as the page
• Search by exact name, including _.

Screenshots

Enviroment:

• OS: [e.g. Fedora 29, Windows 7, etc]
• Browser: [e.g. chrome, safari]

Versions:

• Template: 1.0.0-beta.3
• PHP: 7.2
• Node: 8
• Yarn: 1.13
• VuePress: 0.14.8 | 1.0.0-alpha.32
• phpDocumentor: 2.9.0 | 3.0.0-dev

Posible workaround
Escape _ characters from titles.

Additional context

Report datails in VuePress repo: vuejs/vuepress#1208