Use cobra and viper for configuration

[benzcash]

This PR is designed to use the cobra and viper packages to provide additional configure options.

Changes

• Added base main.go, so go build outputs lightwalletd
• Moved the server startup to the "root" command
• Create a subcommand for version
  Example usage:

./lightwalletd version
lightwalletd version v0.2.0

• Tried to change the current flags as much as possible, but they are double-dashed now (we can add short options if desired)

Flags:
      --bind-addr string         the address to listen on (default "127.0.0.1:9067")
      --cache-size int           number of blocks to hold in the cache (default 80000)
      --config string            config file (default is current directory, lightwalletd.yaml)
  -h, --help                     help for lightwalletd
      --log-file string          log file to write to (default "./server.log")
      --log-level int            log level (logrus 1-7) (default 4)
      --no-tls-very-insecure     run without the required TLS certificate, only for debugging, DO NOT use in production
      --tls-cert string          the path to a TLS certificate (default "./cert.pem")
      --tls-key string           the path to a TLS key file (default "./cert.key")
      --zcash-conf-path string   conf file to pull RPC creds from (default "./zcash.conf")

Added functionality

Provide configuration from file

An example lightwalletd-example.yml is in the PR
Usage:

./lightwalletd --config ./lightwalletd-example.yml
Using config file: /home/bwilson/go/src/github.com/zcash-hackworks/lightwalletd/lightwalletd.yml

Or, rename it to just lightwalletd.yml and it will be auto-discovered

$ ./lightwalletd 
Using config file: /home/bwilson/go/src/github.com/zcash-hackworks/lightwalletd/lightwalletd.yml

Provide configuration from environment variables

The same options can be provided through environment variables
Example Usage:

TLS_CERT=/secrets/lightwallted/cert.pem \
TLS_KEY=/secrets/lightwallted/cert.key \
ZCASH_CONF_PATH=/srv/zcashd/zcash.conf \
CACHE_SIZE=10 \
LOG_FILE=/dev/stdout \
BIND_ADDR=0.0.0.0:9067 \
./lightwalletd

** NOTE ** We can preface these environmental variable with a string like LIGHTWALLETD_ or LWD_ to avoid collisions.

[defuse]

Nice improvement! Looks good modulo the one comment I left about visibly warning when there's no TLS.

[LarryRuane]

Very nice! When I run go build, (or make, same thing), a bunch of changes appear in source-controlled files go.mod and go.sum. Is that just me, or do those files also get modified for you? If so I think those changes should be part of this PR.

[benzcash]

Is that just me, or do those files also get modified for you? If so I think those changes should be part of this PR.

Yeah, forgot them. Added!

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@2687f7d). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
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  Misses            ?      884           
  Partials          ?       57

Impacted Files	Coverage Δ
common/generatecerts.go	0% <0%> (ø)

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[LarryRuane]

Should any changes be made to README.md, especially as relates to deployment, as part of this PR? Perhaps just what's in this PR's first comment.