[Enhancement request]: Stick to patchable version: requires ^= 0.1.0
Currently, nimble only supports `requirements >= version` and `requirements == version` according to the documentation.

Python has some easy requirements.txt [options ](https://pip.pypa.io/en/latest/reference/pip_install/#example-requirements-file) for pip, while node/npm packages.json has some even more complex [syntax](https://docs.npmjs.com/cli/v7/configuring-npm/package-json#dependencies). 

Usually, when trying out python libraries or examples, the most used seems to be the ` == version` operator but there is also a less used but available `~=` operator that only installs patches.
For node/npm, tjhe default seems to be a `^` prefix, which always installs the latest patch/minor version. A  `~` is also available in node but not used that much.
There is an automatic `npm update` command to automatically change the `packages.json`  to the latest version of packages, even for those with `^`  or `~` prefix.

For nim, there is mostly a `>=` operator available, and there is also a new `==` operator for nim versions >= 1.2.
I also found a Nim syntax for requirements in the latest code `>= 1.5 & <=1.8` in /src/nimblepkg/version.nim:parseVersionRange that also seems to work but it is undocumented yet.

I would prefer a simpler npm like syntax:  `^= 1.1.1` which may be converted internally to `>= 1.1.1 & < 2.0.0`  or  `~= 1.1.1` which might be converted to `>= 1.1.1 & < 1.2.0` 
I haven't seen `&` in nim yet with `and` meaning and this actually seems to be a bit complicate for everyday usage.
