Add sample for exec plugin. Fixes #245

[ekini]

No description provided.

[sparrc]

explain where catter comes from as well, I'm assuming this is from name?

Also, while you're here, can you remove the interval section of the README, this is redundant since it's the same for every plugin

[sparrc]

Thanks @ekini! Just a couple comments 👍

[ekini]

1. Replaced "catter" to "mycollector" to make the doc consistent.
2. The plugin doesn't ignore ints, but it treats them as floats. I'm not sure what happens when it passes them to accumulator, and values there are interface{}.
3. As far as I see from the codeI, the interval parameter here is different from the common one.
   For example, if exec.interval = 20, and agent.interval = 10, then the command will be run only once.

[sparrc]

re 3: that's true, I forgot about that, someone added that so you can configure interval on a per-collector basis

re 1 & 2: looks good, thank you!