Changes default search addon behavior

[lramos15]

Changes the default search addon behavior to match what other terminals do. Closes #2339. There are currently no Search Addon test cases but it appears to work in my simple tests.

[Tyriar]

Found an issue, searching "package.json" in this (ll, ls):

-rw-rw-r--   1 daniel daniel    719 Jul 10 11:54 .npmignore
-rw-rw-r--   1 daniel daniel     18 Feb 20 10:10 .npmrc
-rw-rw-r--   1 daniel daniel   2001 Jul 16 14:32 package.json
-rw-rw-r--   1 daniel daniel     38 Feb 20 10:10 Procfile
-rw-rw-r--   1 daniel daniel  14338 Jul 23 12:52 README.md
-rw-rw-r--   1 daniel daniel    402 Jul 10 11:54 tsconfig.all.json
-rw-rw-r--   1 daniel daniel   2676 Jul 16 14:32 tslint.json
-rw-rw-r--   1 daniel daniel   1072 Jul 10 11:54 webpack.config.js
-rw-rw-r--   1 daniel daniel 211264 Jul 16 14:32 yarn.lock
ls
~/dev/Tyriar/xterm.js @pr/lramos15/2340 +5/-8
❯ ls
addons    node_modules  typings              Dockerfile     README.md
bin       out           azure-pipelines.yml  LICENSE        tsconfig.all.json
css       out-test      CODE_OF_CONDUCT.md   logo-full.png  tslint.json
demo      src           CONTRIBUTING.md      package.json   webpack.config.js
fixtures  test          docker-compose.yml   Procfile       yarn.lock

~/dev/Tyriar/xterm.js @pr/lramos15/2340 +5/-8

Will find pack in webpack, but when the second a is pressed it will skip the package.json on the same line.

[Tyriar]

Seems to work 👌