Tabbed session support

I used to use https://github.com/xolox/vim-session, before I started using Startify's ``:S*`` commands to manage sessions.

One benefit of vim-session is their tab scoped sessions: https://github.com/xolox/vim-session#tab-scoped-sessions. That allows opening sessions in a new tab, and closing it would just close the session of that tab & its buffers. One can then then move to other tabs to keep working.

1) Add tabbed session support to Startify, so opening/closing a session in one Vim tab does not affect other tabs. Perhaps you could just use [their implementation](https://github.com/xolox/vim-session#tab-scoped-sessions).

2) Give users a hook to specify custom command/function for "Sessions" on startify page. The hook will default to using the session management functions built into Startify.

Start with Startify's existing session-management functions and compare them with the tab-scoped approach described in the linked vim-session documentation. The work is done when sessions can be opened and closed independently per Vim tab and the Sessions entry supports a user-configurable command or function with the existing management functions as its default.

Written by the indexing model from the issue text.