Remove default <table> styling from the theme css styles

[m-sasashita]

When <table> tags as html with custom styling are used inside .md files,
default styles that are defined inside theme.styl file overrides those custom styles,
making it impossible to use table tags to style custom tables.

Proposal:

Remove css styles that are bound to <table> element inside [/theme/styles/theme.styl] and add a custom class to default tables that are parsed from md syntax.

Steps to reproduce:

1. Use table tags
2. Define the custom styles that overrides the default styles like this:
   th, td {border: none}
   ※the default style:
   th, td {border: 1px solid #dfe2e5}

Expected:

Override the default style by the custom style.

[ulivz]

You can override the default style by using: #app table { } selector.