reorganize a bit; add names.lints, split.lints methods; get lintr to pass most of its own lints :-)

[ttriche]

Hopefully the changes will be fairly obvious. The methods are primarily so that I can run a command like:

lints <- lint_package('lintr')
subdir <- Vectorize(function(x) strsplit(sub("./", "", fixed=TRUE, x), "/")[[1]][1])
by_subdir <- split(lints, subdir(lints))

and get something relatively easy to follow

actually_care_about <- grep('80 characters', vapply(by_subdir$R, `[[`, character(1), 'message'), invert=TRUE)
by_subdir$R[ actually_care_about ]

[[1]]
./R/expect_lint.R:68:3: warning: local variable 'itr' assigned but may not be used
  itr <- 0L
  ^~~

[[2]]
./R/get_source_expressions.R:225:5: style: Closing curly-braces should always be on their own line, unless it's followed by an else.
    })(prev_locs == lag(next_locs))
    ^

[[3]]
./R/lint.R:48:54: style: Place a space before left parenthesis, except in a function call.
        expr_lints <- flatten_lints(linters[[linter]](expr))                                                 ^

[coveralls]

Coverage decreased (-0.23%) to 80.79% when pulling 427e177 on ttriche:master into 734c614 on jimhester:master.

[jimhester]

Looks good to me. Thanks Tim!