Update docs to use underscores

[catsby]

All the commands should be underscores , update docs (and thus heroku help pg output) to reflect

[will]

this is the opposite of #39

[catsby]

Indeed... I had forgotten about 39.

I opened this to consolidate on _. #39 updates the docs to all - but doesn't actually change the methods.

I don't strongly care about _ vs -, as long as documentation and invocation match. The sprinkling of alias_command methods cover this I suppose, if we're OK with that

[msakrejda]

Oh hmm. I thought that the aliases were set up the other way around. It looks like help on the commands now calls them out as aliases:

maciek@gamera:~$ heroku help pg:index-usage
Alias: pg:index-usage redirects to pg:index_usage
Usage: heroku pg:index-usage [DATABASE]

 calculates your index hit rate (effective databases are at 99% and up)

heroku help pg:index_usage shows the same thing but without the alias blurb. This is definitely somewhat confusing, given that the docs treat the dash commands as first-class. Should I swap the aliases?

[will]

Should I swap the aliases?

yes

[msakrejda]

Swapped in #97, so this is obsolete.