link handling and navigation

[scruloose]

This Issue has some overlap with Issue #2, but is broader in scope.

I think the way links are handled in Formiko has a lot of room for improvement. Specifically:

• There's no back button.
  It took me a while to notice that the right-click context menu in the preview pane has navigation options, and they seem to be always greyed out and inactive, even when I have just followed a link. I have tested this with local relative links and remote http:// links, in both two-pane and preview-only modes). The only way I can get the preview pane back to the document I started from is to close the Formiko window and open the document again.
• There's no home button.
  There should be a one-click way to make the preview pane immediately return to the document that's currently open in the edit pane without having to backtrack through a browsing history.
• "Open link in new window" does nothing.
  It doesn't open a new window or follow the link in the existing window.
• There's no option to open links in external browser.
  It would be very helpful to have http:// and https:// URLs passed to my x-www-browser so they open as new tabs in Firefox, instead of having to deal with them in my markdown editor's preview pane.
• Handling of local links is inconsistent
  Links formatted as [link text](file://file.md), [link text](file://./file.md) show up in blue like links, hovertext is the literal URL as-entered, clicking them does nothing. Links formatted as [link text](file:file.md) or [link text](file.md) work as expected, ie they show up in blue, hovertext shows the expanded full path to the file, and clicking opens the target file in the preview pane.
• Relative links to local markdown or plain text files should open the target file for editing.
  I agree with @david_libremone in Issue Using links to navigate between local files #2 that the ideal behaviour when I click on a local link such as ./filename.md is to save & close the current file and immediately open the target file in the same window, keeping the view mode (preview-only vs dual-pane) at its current setting.

[ondratu]

I thing that this issue is done by last pull requests. If you are ok with this, I will close this thicket on release.

[scruloose]

Nice! I look forward to trying the new version.