Add --template flag to inject command

[SimonBarendse]

When using a template from a file (which is the large majority of use-cases), you currently have to cat the file and pipe it to the inject command:

cat env.tpl | secrethub inject

Like with #35, not all environments work nicely with pipes. For example, in a docker image, we prefer a single call to a binary over the shell form of the command.

Therefore, I propose to add a --template-file flag that can be used as an alternative input to stdin.

--template-file (string)

   Use the contents of a file as template. 

And the order in which the command reads the template from different inputs would change to:

1. --template-file
2. If input is piped, read from stdin
3. Error if no other input is given

[mackenbach]

Nice, love how we can make all these forms of input consistent. To be consistent with other commands, consider naming the flag either --template or --input-file (I much prefer --template)

[SimonBarendse]

Yes, let's make it --template, as this is also the name we use for this flag in the run command.

[SimonBarendse]

An --in-file flag is added in v0.24.0.