Allow Initializing in Nonempty Directories

[lkingland]

Currently func [init|create] will error if there are any files in the target directory.

This was a stop-gap measure, since a common pattern would be to first create a git repository, a README, perhaps add some other files, and then run func init [language].

In order to support this very likely workflow, it would be very nice if we could do a pre-apply of the template chosen, tallying up a list of all the files that would be overwritten to give the user a clear error message.

When in Noninteractive Mode (no --confirm):
If there already exists a func.yaml file, the error message would state as such before listing the files and directories
that should be moved or be overwritten with --force:

$ func init go
Target location contains an initialized function.  Use --force to continue, which will overwrite the following files:
func.yaml
handle.go
handle_test.go

If the directory does not contain an initialized function (no func.yaml), the output would be almost identical, just with a slightly differently worded warning:

$ func init go
The target location has files which will be overwritten. Use --force to overwrite the following files:
handle.go
handle_test.go
README.md

When in interactive mode (--confirm), it would use a two-step prompt. Something akin to:

$ func init go --confirm
Target location contains an initialized function.  Continue? (y/N):  y
Choose which files to overwrite? (Choosing 'no' will overwrite all) (Y/n):
Overwrite 'handle.go'? (Y/n)
Overwrite 'handle_test.go'? (Y/n)
Overwrite 'README.md'? (Y/n)

Nuances:

The output enumerating the files to be overwritten need not recursively list everything in .func.
For simplicity's sake, we may want to start by being nonrecursive in general
The --force flag is not something that should be either persisted between runs or be globally configurable
Note how the --confirm mode defaults are to not overwrite an existing function, but if the user does chose to do so, the rest of the defaults are to overwrite.

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