Many twelve-factor apps use environment variables rather than the built in Rails secrets mechanism to store secrets.
The [dotenv](https://github.com/bkeepers/dotenv) gem is widely used for this purpose and allows environment variables to be loaded from the `.env` file. However, this file should not be committed to source control.

This PR just adds the `.env` file to `.gitignore` so that these secrets aren't accidentally committed. In some rare instances, it may be required that this file is committed to source control so I've added it to the list of files that can be removed if the developer is ok with secrets being committed.

Ignore '*.DS_Store' instead of ignore '.DS_Store'

Ignore cabal.project.local

ignore .dSYM along with .ipa files

Add .com.apple.timemachine.donotpresent to OSX

.com.apple.timemachine.donotpresent is a system file created by TimeMachine on OSX by user choice.

Add knitr and R markdown patterns to R.gitignore

These patterns handle cache diretories and temporary files created
by the widely used [knitr](http://yihui.name/knitr/) and [R markdown](http://rmarkdown.rstudio.com/)
packages.

Update MicrosoftOffice.gitignore

Update TeX.gitignore

When we use 'thmtools' package for creating 'List of Theorems', auxiliary file '.loe' is created, just as '.toc' for 'Table of Contents' and '.lof' for 'List of Figures'.

Add .webassets-cache to Flask

Add nyc test coverage output directory

[nyc](https://github.com/bcoe/nyc) is quite popular, and under some circumstances it writes to `.nyc_output` directory. This directory should be ignored by default.

C: Ignore gcc Static Stack Usage Analysis .su files

Add FlashDevelop and executables to Actionscript

Ruby: Ignore .env

- FlashDevelop uses different file and folder names for it's bin and obj outputs, so those have been added for convenience of devs using that.
- Added a list of some executables that are output by various ways of compiling Actionscript (such as AIR and general Flash).

Update Rust.gitignore

This file is used moderately commonly to store environment variables locally. See Python.gitignore.

no need to specifically ignore compiled files and executables
as it will be under the target directory

Added Visual Studio 2015 .vs folder

Haskell: Ignore .eventlog files

Added .ropeproject directory

If anyone uses the ROPE refactoring library this is certainly necessary.

Visual studio new database engine

Ignore files produced by GHCs eventlog framework (https://ghc.haskell.org/trac/ghc/wiki/EventLog)

Ignore session files

Add dist/ folder to Emacs gitignores