fix: report an unknown theme by name instead of crashing

[lenamonj]

rougify highlight -t <unknown-theme> dies with an unhandled NoMethodError instead of the error message the code already contains:

$ rougify highlight -t no-such-theme file.rb
lib/rouge/cli.rb:325:in 'initialize': undefined method 'new' for nil (NoMethodError)

The line is:

theme = Theme.find(opts[:theme]).new or error! "unknown theme #{opts[:theme]}"

.new binds tighter than or, so when Theme.find returns nil the crash happens before or is ever evaluated - the unknown theme message is unreachable. Splitting the lookup from the instantiation makes the intended message fire:

$ rougify highlight -t no-such-theme file.rb
unknown theme no-such-theme

Two lines changed, plus a spec in the existing Rouge::CLI::Highlight block asserting the Rouge::CLI::Error message. With the fix reverted that spec errors with the original NoMethodError; with it, rake check:specs is green at 1325 runs / 5804 assertions.