Set minimum version in use_package()

I mostly require to have package version next to the package name in the `DESCRIPTION` file.  An additional argument such as `add.version` to the function would be quite useful. For example:

```r
usethis::use_package(package = "dplyr", type = "Imports", add.version = TRUE)
```
Then, it will be adding 'dplyr' to Imports field in DESCRIPTION:
```
Imports: 
    dplyr (>= 0.7.5)
```
