Rule: no-lonely-if (fixes #790)

Adds a new rule, no-lonely-if, which warns whenever an if statement
is the only statement inside the else block of another if statement.
The rule is off by default.
Adds `space-after-keywords` rule (fixes #807)

Enforces spacing after keywords such as 'if', 'do', 'for', and so on.
Remove warning about ESLint being in Alpha
Merge pull request #808 from spadgos/space-after-keywords

Adds `space-after-keywords` rule, along with associated tests and docs
Merge pull request #817 from spadgos/remove-alpha-warning

Remove warning about ESLint being in Alpha
Merge pull request #816 from btmills/no-lonely-if

Rule: no-lonely-if (fixes #790)
Ignored files are excluded unless --force is passed on the CLI
Ignored files are reported with a warning