Add CI starter workflows

fixes #782

Author: ivotron

examples/README.md

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+# Examples
+
+This folder contains examples on how to make use of Popper in distinct 
+contexts:
+
+  * [Continuous Integration (CI)](./ci/). A list of starter workflows 
+    for using Popper to continuously test your project.

examples/ci/README.md

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+# CI workflow starters
+
+A list of examples that can be used as starter points for automating 
+CI workflows with Popper.
+
+## CI Service Setup
+
+You can use the `popper` tool to generate configuration files that are 
+consumed by a CI service to automatically run a workflow. Assuming you 
+place the starter workflow on the root of your repository, you can 
+generate a CI configuration file by doing:
+
+```bash
+popper ci --file ci.yml <SERVICE>
+```
+
+Where `SERVICE` is the name of a supported service (Travis, CircleCI, 
+Jenkins and Gitlab are supported). See more in [Popper 
+documentation](../docs/sections/cli_features.md#continuously-validating-a-workflow).

examples/ci/cpp-conan.yml

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+steps:
+- id: dev-init
+  uses: docker://conanio/gcc72:1.25.2
+  runs: [bash]
+  args:
+  - -c
+  - |
+    rm -rf build
+    mkdir -p build
+    cd build
+    conan install ../ --build=missing
+    cmake -G Ninja -DCMAKE_BUILD_TYPE=Release ..
+
+- id: build
+  uses: docker://conanio/gcc72:1.25.2
+  runs: [bash]
+  args:
+  - -c
+  - |
+    cd build/
+    cmake --build .
+
+- id: test
+  uses: docker://conanio/gcc72:1.25.2
+  runs: [bash]
+  args:
+  - -c
+  - |
+    cd build/
+    make test

examples/ci/go.yml

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+steps:
+- id: get deps
+  uses: docker://golang:1.14.4-alpine3.11
+  args: [go, get, -v, -t, -d, ./...]
+
+- name: build
+  uses: docker://golang:1.14.4-alpine3.11
+  args: [go, build, -v, .]
+
+- name: test
+  uses: docker://golang:1.14.4-alpine3.11
+  args: [go, test, -v, .]

examples/ci/nodejs.yml

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+steps:
+- id: install
+  uses: docker://node:12-alpine
+  args: [yarn, install]
+
+- id: build
+  uses: docker://node:12-alpine
+  args: [yarn, build]
+
+- id: test
+  uses: docker://node:12-alpine
+  args: [yarn, test]