`command add` should not override content

[jvalkeal]

We should simply fail if .spring/commands/hello/new exists.

15:02 $ spring command new
Created user defined command: /home/jvalkealahti/repos/jvalkeal/spring-cli/dogfood/.spring/commands/hello/new

15:02 $ ls -lt .spring/commands/hello/new
total 8
-rw-rw-r-- 1 jvalkealahti jvalkealahti 231 Apr 26 15:02 command.yaml
-rw-rw-r-- 1 jvalkealahti jvalkealahti  90 Apr 26 15:02 hello.yaml

15:03 $ spring command new
Created user defined command: /home/jvalkealahti/repos/jvalkeal/spring-cli/dogfood/.spring/commands/hello/new

15:03 $ ls -lt .spring/commands/hello/new
total 8
-rw-rw-r-- 1 jvalkealahti jvalkealahti 231 Apr 26 15:03 command.yaml
-rw-rw-r-- 1 jvalkealahti jvalkealahti  90 Apr 26 15:03 hello.yaml