how to use an environment variable in a path to a file in an annotation
Hi. 

I am using taskopen ``v1.1.5 297 b9721f5`` in two different environments:
 
* macos Big Sur
* Debian 10 running  virtually in chromeos

I would like to use taskopen to manage not only http links and Notes links, but also links to my own markdown files, which I maintain in vimwiki. In order for this to work in macos, I had to set ``FILE_CMD`` to ``vim``: in macos, the vanilla open command doesn't know how to open ``.md`` files. 

I use my markdown files on both systems and have an extensive set of environment variables to make things work in both places.

The only problem with the annotations as I now add them is that they use an absolute hard coded path, for example:

```
/Users/docs-base/mkdn/03_tchn/sysi/mylatest_file.md
```

The actual path on my second system will be different, and so opening these annotations on the other system will not work without using an environment variable in my file paths. 

I tried to solve this problem by creating an alias function to annotate tasks with these file links: it writes the path to the file with the correct environment variable. I then end up with an annotatiion that uses a path like this:

```
$MKDN_ROOT/03_tchn/sysi/mylatest_file.md
```
In order for taskopen to recognize this as a legit file link, I had to modify ``FILE_REGEX`` as follows:

```
 FILE_REGEX="\$|\.|\/|~" 
```
When I try to open this link in taskopoen, it doesn't expand the environment variable: it is looking for a directory that is literally named ``$MKDN_ROOT``

When I used ``taskopen -L`` to see how it would open the link, I noticed that my link was enclosed in single quotes whereas the links to Notes -- which use environment variables quite handily -- are in double quotes. I experimented with various means of making my links have doouble quotes (escaping double quotes, escaping two double quotes) all to no avail: taskopen appears to be quite insistent on single-quoting these links. 

I am hoping that I am overlooking something simple in all this (I'm quite new to shell scripting) or that there is a different solution through configuration. 

Thanks so much -- I love this tool!
--Phil
