Make it possible to open .md files with ation from system #2

We would like to have a more refined experience in this matter, so the app feels a lot more native. The enhancements include:

being able to select ation in the os' "open with" context menu and open files that way
being able to drag and drop a file into ation's main window
being able to open .md files with ation from the command line

make the opening of files asynchronous between renderer and backend process, so either one can open files
register ation as a handler for markdown files
implement a onDrop handler in the main window
parse command line arguments for console access; make a /usr/local/bin binding for ation and something similar for windows (maybe later)
add "File -> open" menu item to main menu

parse command line arguments for console access; make a /usr/local/bin binding for ation and something similar for windows (maybe later)

this does not to seem possible without making an installer. for now, the user should make an alias himself.