[Bug] Can't use environment variable with some path value on Windows

[Fiery-Fenix]

Bug report

Relevant telegraf.conf:

[agent]
  interval = "10s"
  flush_interval = "10s"
  round_interval = false
  flush_jitter = "1s"
  collection_jitter = "1s"
  metric_batch_size = 1000
  metric_buffer_limit = 10000
  debug = false
  quiet = false
  logfile = "$TEMP\\telegraf.log"

System info:

Telegraf v1.5.0 (git: release-1.5 a1668bb)
OS Name: Microsoft Windows 10 Enterprise
OS Version: 10.0.16299 N/A Build 16299
TEMP Environment variable value: TEMP=C:\Temp

Steps to reproduce:

Use provided config file and try to run telegraf:

telegraf.exe --config telegraf.conf --test

Expected behavior:

Telegraf runs and log file created in path C:\Temp\telegraf.log

Actual behavior:

2017/12/20 17:36:46 E! Error parsing telegraf.conf, toml: line 11: parse error

Additional info:

This is because environment variable (including system variables) contains only one slash in path, but for accessing such path from some system commands (open/read/write/etc.) you need to double slashes.
Current schema:

1. Read config file
2. Replace environment variables with their values
3. Parse TOML

Proposed schema:

1. Read config file
2. If OS==Windows and environment variable value contains single slash - replace it with double slash, other variables replace as usual
3. Parse TOML