[ADDED] Support a path as argument to --signal

[pires]

• Link to issue, e.g. Resolves #NNN
• Documentation added (if applicable)
• Tests added agreed w/ @kozlovic test is not mandatory for this one.
• Branch rebased on top of current master (git pull --rebase origin master)
• Changes squashed to a single commit (described here)
• Build is green in Travis CI
• You have certified that the contribution is your original work and that you license the work to the project under the Apache 2 license

Resolves #

Changes proposed in this pull request:

• Make the -sl / --signal flag accept the path to a PID file as an argument. This is useful in scenarios where pgrep isn't available (such as in the official Docker images) but there is a PID file.

/cc @nats-io/core

[kozlovic]

LGTM! Thank you for the contribution

[derekcollison]

LGTM

[coveralls]

Coverage increased (+0.003%) to 91.627% when pulling 67412b7 on pires:pires/ldm_pidfile into afc3a45 on nats-io:master.