add .lintr config and run lintr on Travis (.com)

.lintr

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+linters: with_defaults(
+    # lintr defaults: https://github.com/jimhester/lintr#available-linters
+    # the following setup changes/removes certain linters
+    assignment_linter = NULL, # do not force using <- for assignments
+    object_name_linter = object_name_linter(c("snake_case", "CamelCase")), # only allow snake case and camel case object names
+    cyclocomp_linter = NULL, # do not check function complexity
+    commented_code_linter = NULL # allow code in comments
+    line_length_linter = line_length_linter(100)
+    )

.travis.yml

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+# OS ---------------------------------------------------------------------------
+os: linux
+dist: bionic
+
+# meta -------------------------------------------------------------------------
+language: r
+cache: packages
+latex: false
+
+# Stages -----------------------------------------------------------------------
+
+install: true
+script:
+  - Rscript -e 'if (!requireNamespace("lintr")) install.packages("lintr")'
+  - Rscript -e 'lintr::lint_package()'