Implement "Reopen with Encoding" command in File menu

[yhatt]

It has mentioned in #22. I added Reopen with Encoding command to File menu.

• Reopen with Encoding is disabled in new document, and it's enabled in existed file.
• When you click menu-item of the encoding, an opened file will re-open from disk.
• If you have edited buffer, Marp asks whether re-opening.

Implemented encoding which have organized menu-items by country groups are based on atom/encoding-selector.

Screen cast

[yhatt]

Additional: Selected encoding doesn't apply on save because we had planned to use encoding UTF-8 for save.

[will-hart]

Will this also address #39 and #46?

[yhatt]

No. These issue was brought by the rendered font as said on #39. I'll create another pull request for these.

[DivineDominion]

Fixes #73! Great.

Is there no way to detect the file's encoding and use it by default?

[yhatt]

Actually, Marp already detects text encoding whenever opening file. (Powered by jschardet)

But it has edge-cases to fail when text file encoded by UTF-8 includes umlauts. This feature made for deal with it. Please see #22 and #59.