Update readme.md
Fix the bug that can't delete the session item.
Add new option "When this extension was updated, If your purging tabs have closed, the extension restores session promptly.".
When the extension was initialized, fix the bug that already the opened purging tabs can't add to the variable of unloaded.
Modified the codes that process context menu.

To brevity and simple.
Fixed the error to occur when run restore function.
The process initialize has improved.

Each process is split into functions.
Change default config.
Fix the bug (Issues #43)

Fix that There are times when don't save sessions correctly.