Use link only when text is selected

* Add page_url variable which always expands to the page URL.
* Prefer the selected text to the link text.

Fix the error for not being copied with context menus.

Add the option to copy as HTML as well as Text.
In the options page, check HTML checkbox for the format you want to copy as HTML.

Fix using default options when no settings in storage

Fix for parse error handling

Use new clipboard.WriteText API.
Now you don't have to set dom.event.clipboardevents.enabled in about:config to true.

Nothing changed. Uploaded just to get signed on addons.mozilla.org

Added keyboard shortcuts!
Also improve the way to get the first link in the selection if some text is selected when you copy a link using keyboard shortcut, context menu, or toolbar button.

Use CR+LF for newline on Windows.
Newlines (\n) works now when you paste URLs to notepad.exe.

Revert the context menu target is limited to link, page, and selection.
Now you can use the context menu on images again.

No UI change.
An internal code cleanup.

Added a checkbox "Create submenus" in the options page.

If you check the "Create submenus" in the options page and save the options,
submenus for each format are created under the "Format Link" context menu group.

Version 2.0.0!
This is a major version up, and the UI is greatly simplified.

Now you can use only the context menu to copy the URL.

1. When you open the menu over a link, the link title and the link URL are copied.
* However, due to the WebExtensions API limitation, the text is taken from the first link of the same URL in the page,
so it may be different from what you want.
2. When you select the text and open the menu over a link, the selected text and the link URL are copied.
* Note you can select the text and move the mouse cursor to a link which is outside of the selection.
3. When you open the menu somewhere not over a link, you can copy the page URL.
* The text becomes the selected text if you selected some text, it becomes the page title otherwise.

The previous version has multiple context menu items for each format to copy.
The version 2.0.0 has only one context menu item.
This change was made to copy a link in shorter steps.

You can select the format with the radio button on the popup of the toolbar button.
When you select the radio button, it becomes the default format.
The "Set as default" button in the previous version was deleted.

Sorry for inconvenience with this major UI changes,
but I believe this version 2.0.0 is simpler and easier to use.
I hope you are accustomed to the new UI soon!

Fixed setting the default format with the context menu with Ctrl key pressed.

Rename context menu item labels

When you right click on a link without selecting text manually, {{text}} becomes the text of the first link of the same URL.
If there is another link of the same URL, {{text}} may be different than the text of you actually selected.

This is a workaround for https://bugzilla.mozilla.org/show_bug.cgi?id=1325814

Now you can get the link text without selecting manually when you use the context menu on a link.

Use page title when getSelection fails.

Add keyboard shortcut Alt+C to copy the URL in the default format.